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<article id="post-Java-I-O" class="article article-type-post" itemscope="" itemprop="blogPost">
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<a href="/2020/07/02/Java-I-O/" class="article-date">
<time datetime="2020-07-02T13:35:12.000Z" itemprop="datePublished">2020-07-02</time>
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<a class="article-title" href="/2020/07/02/Java-I-O/">Java-I/O</a>
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<p>#BIO<br>InputStream、OutputStream(基于字节流，读取二进制文件)文件流、数据流、对象流<br>Reader、Writer（基于字符流，读取文本文件）</p>
<p>传统I/O是阻塞式的，当我们调用reader或者writer的时候，再有数据可以读取前，线程都是被阻塞的<br>NIO可以在等待数据时，让线程不阻塞，继续做其他的事情</p>
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<article id="post-Android提升" class="article article-type-post" itemscope="" itemprop="blogPost">
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<a class="article-title" href="/2020/07/01/Android提升/">Android提升</a>
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<p>#Handler</p>
<ol>
<li>在非ui线程中更新ui会抛出calledFromWrongThreadException</li>
<li>Android为了线程安全，并不允许非ui线程去更新ui</li>
<li>handler是一套用来更新ui、处理消息的机制</li>
<li>所有activity生命周期的函数都是通过handler去发送消息的，根据不同的message做不同的分支处理</li>
<li><p>不遵循handler就无法更新ui，Android已经封装好了消息的创建、传递、处理机制<br>关键点：更新ui、处理消息（发送/接收）</p>
</li>
<li><p>我们在创建Handler时，它会和一个默认的线程绑定，在这个默认的线程中会有一个MessageQueue</p>
</li>
<li>ui线程会创建一个looper</li>
</ol>
<p>#Handler用法</p>
<ol>
<li>ui线程创建handler实例，重写handlerMessage方法,在该方法中更新ui；在子线程中创建Message方法，用handler对象去sendMessage</li>
<li>在ui线程中创建Hanlder实例，在子线程中用handler.post(new Runnable)的方式,在run方法去更新ui<br>ps:接口和抽象类是不能进行实例化的，实际上是一种内部类的简写。java创建了一个实现Runnable接口的无名内部类</li>
<li>利用post.delayed方法在子线程中更新ui线程(Runnable,时间)，实现一个倒计时的界面；自动切换页面；定时更新图片<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">MyRunnable myRunnable = new <span class="function"><span class="title">MyRunnable</span></span>(){</span><br><span class="line"> @Override</span><br><span class="line"> public void <span class="function"><span class="title">run</span></span>(){</span><br><span class="line"> index++;</span><br><span class="line"> index=index%3;</span><br><span class="line"> imageView.setImageResource(images[index]);</span><br><span class="line"> handler.postDelayed(myRunnable,time);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">在onCreate()中</span><br><span class="line">handler.postDelayed(Runnable,time);</span><br><span class="line"></span><br><span class="line">Message message = new Message();</span><br><span class="line">或者 Message message = handler.obtainMessage();//查看是否有空的message可复用，如果没有，则new出一个新的</span><br><span class="line">message.arg1=88;</span><br><span class="line">message.obj=student;</span><br><span class="line">handle.sendMessage(message);</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>#Handler原理</p>
<ol>
<li>作用：更新ui、处理消息</li>
<li>目的：解决多线程并发问题</li>
<li>线程安全问题：临界资源+多线程操作<br>在不加锁的情况下，多个线程更新ui会导致界面错乱<br>在加锁处理的情况下，将导致程序性能下降</li>
<li>所以使用Handler机制，所有更新ui的操作，都在主线程的消息队列中进行轮询处理</li>
</ol>
<p>#handler、Looper和MessageQueue的关系</p>
<ol>
<li>Looper的内部包含一个MessageQueue，用来存储消息,在UI线程中</li>
<li>Handler在构造时会和Looper进行关联</li>
<li>Handler负责发送Message（sendMessage）给MessageQueue</li>
<li>Looper从MessageQueue中取出待处理的消息，回发给Handler，执行handleMessage的方法</li>
<li>Looper接收到Handler发送的消息，并把消息回发给Handler(handleMessage)</li>
</ol>
<p>#ActivityThread</p>
<ol>
<li>在ActivityThread中创建Activity，以及调用回调方法，默认创建一个线程（main线程、ui线程）</li>
<li>在ui线程中会创建looper,looper会创建messageQueue</li>
</ol>
<p>#自定义控件</p>
<ol>
<li>目的：为了达到特别的显示效果、特色功能<br>步骤1：在attrs.xml中定义声明<br>步骤2：在layout xml中使用<br>步骤3：编写继承自已有控件的Java类，内部编写控件逻辑</li>
</ol>
<p>#布局优化<br>Activity->fragment</p>
<ol>
<li>include优化标签<br>在一个布局中引入另一个布局，方便布局文件统一风格，减少开发和维护时的工作量（引入公共部分，统一的导航栏、广告栏，类似于import）</li>
<li>merge<br>减少布局嵌套层次，提高布局加载效率，是include的辅助扩展<br>将引入布局的根节点改成merge标签，减少布局的嵌套</li>
<li>viewstub<br>用viewstub引入一个布局，只有加载该布局时，才占用资源，不可见的控件是不会绘制出来的<br>不可见的情况下，是不会加载这个资源的，适合用于报告网络异常状况<br>不可见，不加载</li>
</ol>
<p>#内存优化<br>内存泄漏：申请的内存没有正确被回收，导致这部分内存自身无法使用，别的程序也无法使用。如果不加以控制，程序所占用的内存将会越来越大，最终导致死机。<br>原因：</p>
<ol>
<li>Java Native Interface，C/C++编写</li>
<li>调用来外部服务</li>
</ol>
<p>Bitmap对象要及时recycle<br>注册了Android系统的服务，要及时注销。LocationManager、PowerManager</p>
<p>#View的绘制过程</p>
<ol>
<li><p>测量尺寸<br>重写onMeasure<br>View测量自身，决定自身所占范围<br>由mode和值确定<br>三种MODE:<br>EXACTLY:明确规定<br>AT_MOST: 至多不能超过某个值<br>UNSPECIFIED：没有限制</p>
</li>
<li><p>确定View位置<br>重写onLayout,决定子View的位置<br>定义类似于RelativeLayout、LinearLayout的显示规则</p>
</li>
<li><p>绘制View<br>重写onDraw,绘制出内容区域</p>
</li>
</ol>
<p>#Retrofit</p>
<ol>
<li>一个网络请求库，以okHttp框架作为httpclient</li>
<li>通过注解简化网络请求 GET PUT</li>
<li>网络请求转化成Interface的形式</li>
<li>在接口中编写请求方法</li>
<li>使用时创建Retrofit对象，执行请求方法，获得返回对象</li>
</ol>
<p>#多进程 VS 多线程<br>王牌金句：进程是资源分配的最小单位，线程是CPU调度的最小单位</p>
<ol>
<li>多进程鲁棒性更高，线程挂了进程也挂了</li>
<li><p>多进程可以进行并行多核的计算（python）</p>
</li>
<li><p>数据共享方面：多线程利于数据的共享</p>
</li>
<li><p>切换：多线程更快，线程没有独立的内存地址，不需要进行内存地址的转换</p>
</li>
<li><p>需求，推送进程</p>
</li>
</ol>
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<article id="post-KMP算法" class="article article-type-post" itemscope="" itemprop="blogPost">
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<time datetime="2020-06-30T09:21:57.000Z" itemprop="datePublished">2020-06-30</time>
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<a class="article-title" href="/2020/06/30/KMP算法/">KMP算法</a>
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<p>#算法本质流程<br>在主串和子串匹配的过程中，如果发现又不匹配的字符，则去检查字串已经匹配过的部分。<br>如果已经匹配过的部分存在公共前后缀，则从前缀后面的那一个字符开始接下来的匹配<br>如果不存在公共前后缀，则从头开始</p>
<p>#算法流程</p>
<ol>
<li>构建next数组<br>next[0]肯定为0；<br>从索引1开始到最后一个，如果相等的话，next[j]值就在上一个基础上+1<br>如果不等的话，next[j]的值就变成和next[j-1]作为索引的元素去比较</li>
</ol>
<p>创建一个和char数组p长度一样的int类型next数组<br>初始化两个指针,next[0]<br>循环硬背流，条件是数组长度减一，内部条件是k初值或jk元素相等<br>相等则移动指针,用k作为当前next[j]的值<br>不相等则用next[k]的值来作为新的k值<br><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">int k=-1,j=0;</span><br><span class="line">next[0]=-1;</span><br><span class="line"><span class="keyword">while</span>(p<p.length-1){</span><br><span class="line"> <span class="keyword">if</span>(k==-1||p[k]==p[j]){</span><br><span class="line"> k++;</span><br><span class="line"> j++;</span><br><span class="line"> next[j]=k;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span>{</span><br><span class="line"> k = next[k];</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">return</span> next</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>利用next数组进行匹配<br>i,j初始化都为0。i是源串指针，j是模式指针<br>最外层循环时两个串都不越界<br>内部条件是j==-1或者对应字符相等<br>这样就移动两个指针<br>否则，将j指针更新为next[j]值</p>
<p>结束部分是如果j（模式串指针）等于模式传长度，则返回两个指针之差<br>否则返回-1<br><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">while</span>(i<pattern.length && j<source.length){</span><br><span class="line"> <span class="keyword">if</span>(j==-1||pattern[j]==<span class="built_in">source</span>[i]){</span><br><span class="line"> i++;</span><br><span class="line"> j++</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span>{</span><br><span class="line"> j=next[j];</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(j==pattern.length){</span><br><span class="line"> <span class="built_in">return</span> i-j;</span><br><span class="line">}</span><br><span class="line"><span class="keyword">else</span>{</span><br><span class="line"> <span class="built_in">return</span> -1</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>String对象快速转成char[]数组<br><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">char[] array = str.toCharArray();</span><br></pre></td></tr></table></figure></p>
<p>String类型的indexOf这个函数可以完全取代KMP</p>
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<p>#Java中的排序<br><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">int[] arr={3,2,1};</span><br><span class="line">Arrays.sort(arr);</span><br></pre></td></tr></table></figure></p>
<p>转String:String.valueOf()<br>转Integer:Integer.parseInt()<br>Math.random返回一个[0,1)之间的double值</p>
<p>#优先队列<br>插入的元素会自动排序，非常适合于找到第k小的数字<br><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">Queue<Integer> queue = new PriorityQueue<>();</span><br><span class="line">//加入(O1,O2)->O2-O1表示从大往小排</span><br><span class="line">Queue<Integer> queue = new PriorityQueue<>((O1,O2)->O2-O1)</span><br><span class="line">queue.add()</span><br><span class="line">queue.poll()</span><br><span class="line">queue.peek()</span><br><span class="line">queue.size()</span><br></pre></td></tr></table></figure></p>
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<p>#线程与进程的由来</p>
<ol>
<li>串行：需要长时间等待用户输入</li>
<li>批处理：将用户指令集中成任务清单，批量串行处理，但是无法并行（等待I/O操作的过程中，还是无法让出CPU资源）</li>
<li>多进程：进程独占内存空间，保存各自运行状态，相互之间不干扰，且可以互相切换，为并发处理提供可能。能够够在多个进程之间切换时间片。</li>
</ol>
<p>后来人们对实时性有了要求。一个进程可能具有很多的子任务，而进程只能一个个地去执行这些子任务。而往往子任务之间不存在顺序上的依赖，可以并发执行，所以将CPU切分成更细的时间片去执行任务。</p>
<p>同一个进程中的线程共享进程内的资源，相互切换更加快速，因为不需要页目录改变内存地址</p>
<p>#金句：进程是资源分配的最小单位，线程是CPU调度的最小单位</p>
<ol>
<li>与进程相关的资源，都被记录在了PCB中</li>
<li>进程是抢占处理机的调度单位，有自己的内存地址空间；线程属于某个进程，与进程内的其它线程共享进程的内存地址空间</li>
<li>线程由堆栈寄存器、程序计数器、TCB构成</li>
</ol>
<p>#线程与进程的区别</p>
<ol>
<li>线程不能看做独立的应用，进程可以看作是独立的应用</li>
<li>进程有独立的地址空间，相互不影响，线程只是进程的不同执行路径（某个线程挂掉，进程也挂掉）</li>
<li>线程有自己的堆栈、局部变量，但是线程之间没有单独的地址空间。所以多进程程序比多线程程序健壮。</li>
<li>进程切换比线程切花开销大。对于需要同时进行，又要共享某些变量的并发操作，只能用线程。</li>
</ol>
<p>#Java中的进程和线程<br>Java对操作系统中的进程和线程进行了进一步的封装</p>
<ol>
<li>运行一个Java程序产生一个进程，进程中至少包含一个线程</li>
<li>每个进程对应一个JVM实例</li>
<li>Java采用单线程编程模型，程序会自动创建主线程。但JVM中并不是只有主线程一个线程，比如还有GC线程</li>
<li>一个程序是一个可执行的文件，一个进程是一个程序运行中的实例</li>
</ol>
<p>#run vs start</p>
<ol>
<li>用Thread对象调用run还是在主线程中执行，用Thread对象调用start则是在子线程中执行</li>
<li>start会创建一个新的线程去执行run方法；run是直接对普通方法的调用，还是在主线程中执行</li>
</ol>
<p>#Thread VS Runnable</p>
<ol>
<li>Thread是一个实现了Runnable接口的类；Runnable接口是一个接口，里面只有一个抽象的run方法</li>
<li><p>使用Thread创建线程</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">Thread t1 = new <span class="function"><span class="title">Thread</span></span>(){</span><br><span class="line"> @Override</span><br><span class="line"> public void <span class="function"><span class="title">run</span></span>(){</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line">MyThread t2 = new MyThread();</span><br></pre></td></tr></table></figure>
</li>
<li><p>使用Runnable接口构造线程</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">MyRunnable r1 = new MyRunnable();</span><br><span class="line">Thread t1 = new Thread(r1);</span><br></pre></td></tr></table></figure>
</li>
<li><p>Java单一集成，为了提升可扩展性，尽量使用接口的方式创建线程，将业务逻辑封装在run中</p>
</li>
<li>Thread本身就是一个实现了Runnable接口的类，类本身还包含了一些多线程的配套特性</li>
<li>给run方法传参的方式：通过类中的其它数据域，如构造函数、属性等<br>ps：在main中能够使用自身非静态成员时，可以通过在main中创建对象去引用，没必要都去使用static</li>
</ol>
<p>Thread类的方法：sleep（类）、join（Thread对象）</p>
<p>#处理线程的返回值</p>
<ol>
<li>主线程等待发<br>用一个whie循环+线程sleep的方式不断地去等待子线程完成返回值，无法做到精准控制</li>
<li>使用Thread类的join“阻塞当前线程”，以等待子线程执行完毕<br>比主线程等待法精确一些，但无法做到更精准</li>
<li>通过Callable方法实现：FutureTask or 线程池<br>FutureTask中的isDone方法可以判断call方法有没有执行完成实现return<br>FutureTask中的get方法可以在有限时间内等待call方法传来的返回值</li>
</ol>
<p>线程池<br>原理类似，通过future类的两个方法isDone和get去判断和等待返回值<br>使用线程池能够实现多个callable对象的提交，并实现统一管理</p>
<p>ps：public static void main的由来<br>Java中的方法一般都需要创建一个对象，然后用对象去调用；但是主方法是交给JVM直接去调用的，是交给JVM的执行入口，所以需要使用static</p>
<p>#Java线程的状态</p>
<ol>
<li>新建New：创建了线程但是并没有利用start启动的状态</li>
<li>运行Runnable：包括了操作系统中的Ready和Running两个状态</li>
<li>无限期等待Waiting：不会被分配CPU执行时间，需要显示唤醒，否则无限等待<br>e.g. Object.wait()使用默认参数时、Thread.join使用默认参数时，都会造成无限等待</li>
<li>限期等待Timed Waiting：在一定时间后会由系统自动唤醒<br>e.g.带参的wait和join方法、sleep</li>
<li>阻塞Blocked:等待获取排它锁（在有其它线程放弃锁时会发生），等锁状态。不同于等待状态，等待状态并不是因为获取不到锁而导致的。</li>
<li>终止Terminated:线程结束执行</li>
</ol>
<p>#sleep VS wait</p>
<ol>
<li>sleep是Thread类的方法；wait是Object类都有的方法</li>
<li>sleep方法可以在任何地方使用；wait只能在sychronized块或者sychronized方法中使用(获取锁了才能去释放锁)</li>
<li>Thread.sleep只是让出CPU，不会导致锁行为改变</li>
<li>wait不仅让出CPU，而且会释放锁<br>定义对象锁<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">final Object lock = new Object();</span><br><span class="line">sychronized{</span><br><span class="line"></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>无限等待的wait要用notify或者notifyAll()去唤醒</p>
<h1 id="notify-VS-notifyALL"><a href="#notify-VS-notifyALL" class="headerlink" title="notify VS notifyALL"></a>notify VS notifyALL</h1><p>前置知识：锁池EntryList、等待池WaitList</p>
<ol>
<li>锁池：没有抢到锁的线程在锁池中等待锁的释放</li>
<li>等待池：锁的拥有者在用wait方法释放锁后，会进入到等待池中，但是不会参与到锁的竞争中</li>
<li>notifyAll会让所有处于等待池的线程全部进入到锁池中去一起竞争锁</li>
<li>notify只会随机选取一个处于等待池中的锁进入到锁池中去竞争锁</li>
</ol>
<p>#sychronized<br>表明必须获得同步锁，才能执行内部的逻辑</p>
<p>#volatile<br>当有多个线程修改volatile变量时，一旦A线程修改了变量，其它的线程都能立即看到改动</p>
<p>#yield</p>
<ol>
<li>yield函数可以使当前线程给线程调度器一个暗示，表示当前线程愿意让出当前的CPU资源。但是决定权在线程调度器，线程可能会忽略掉这个暗示。</li>
<li>和sleep都是Thread的方法</li>
<li>yield对锁的行为不会产生影响，不会让锁</li>
</ol>
<p>#interrupt<br>中断线程的方式，类似于yield，只是给线程一个中断的暗示</p>
<ol>
<li>对于阻塞状态的线程，使用interrupt方法可以让它立刻终止，并抛出interruptException</li>
<li>对于正常运行状态的线程，只是将线程的中断标志设置为true。</li>
</ol>
<p>#线程安全问题原因</p>
<ol>
<li>存在共享数据</li>
<li>存在多条线程共同操作这些数据<br>解决方法：统一时刻有且只有一个线程在操作共享数据，其它线程需要等待</li>
</ol>
<p>#互斥锁特性<br>原子性：同一时间只允许一个线程进行操作<br>可见性：保证共享数据的变化可以被其它数据看到<br>synchronize保证了原子性和可见性</p>
<p>#synchronized获取对象锁</p>
<ol>
<li>同步代码块，以this指针或者类实例对象作为参数</li>
<li>同步非静态方法，锁是当前对象的实例对象</li>
</ol>
<p>#sychronized获取类锁（制约同一个类的不同实例）</p>
<ol>
<li>同步代码块，以类对象作为参数</li>
<li><p>同步静态方法<br>#Reentrantlock(再入锁)</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">ReetrantLock lock = new ReetrantLock();</span><br><span class="line">lock.lock();</span><br><span class="line"></span><br><span class="line">lock.unlock();</span><br></pre></td></tr></table></figure>
</li>
<li><p>本质：sychronized是关键字，ReetrantLock是一个类（有方法、有属性、可以扩展）</p>
</li>
<li><p>释放锁的方式<br>sychronized会在它执行的部分执行完毕后自动释放锁<br>ReentrantLock使用lock()加锁，使用unlock()释放锁，</p>
</li>
<li><p>精密度<br>控制得更加精确，<br>Reetrantlock还可以对等待锁的时间进行控制</p>
</li>
<li><p>公平非公平<br>sychronized使用的是非公平锁，抢占顺序不一定，有运气成分<br>Reetrantlock可以在构造函数中设置true参数，来得到公平锁。公平锁可以把所有先交给等待时间最久的线程。</p>
</li>
</ol>
<p>ReetrantLock将所转变成了可控的对象</p>
<p>#线程池</p>
<ol>
<li>问题：服务器接收大量的并发请求，没有线程池的情况下，线程需要频繁的创建和销毁，造成大量的开销。</li>
<li>作用：线程池能够帮助我们重复利用线程，去完成新的任务</li>
<li>使用Executors创建线程池</li>
<li>几种类型的线程池<br>newFixedThreadPool(int) n-n<br>指定线程数量的线程池<br>newCachedThreadPool 0-Integer.MAX_VALUE<br>缓存线程，长时间不用就遗弃。这样能在高峰时实现服用，闲置时不占用资源</li>
</ol>
<p>将任务分发给线程池中的线程</p>
<p>#Callable<br>Callable弥补了Runnable无法返回结果的短板</p>
<p>多进程和多线程都可以实现并行</p>
<p>HashMap并不是线程安全的</p>
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<p>#最小未出现的正整数</p>
<ol>
<li>创建一个和当前数组相同的数组</li>
<li>使用for+while+swap实现一个换座位的算法</li>
<li>第一个人与它应该做的座位的人交换位置，然后重复操作</li>
<li>对之后的每个座位执行同样的操作</li>
<li>如果前面几个人刚好是前面的几个正整数的话，应该正好“对号入座”。即nums[i]应该在nums[nums[i]-1]这个位置上</li>
<li>如果不是，则有人的值与索引值不一致<br>巧妙利用了索引与正整数的对应关系<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line">public int firstMissingPositive(int[] nums) {</span><br><span class="line"> int temp = 0;</span><br><span class="line"> <span class="keyword">for</span>(int i=0;i<nums.length;i++){</span><br><span class="line"> <span class="keyword">while</span>(nums[i]>0 && nums[i]<=nums.length && nums[i] != nums[nums[i]-1]){</span><br><span class="line"> //交换nums[i]和nums[nums[i-1]]</span><br><span class="line"> swap(nums,i,nums[i]-1);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">for</span>(int i=0;i<nums.length;i++){</span><br><span class="line"> <span class="keyword">if</span>(nums[i]!=i+1){</span><br><span class="line"> <span class="built_in">return</span> i+1;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> //此时说明数组中正好是正整数的前几位</span><br><span class="line"> <span class="built_in">return</span> nums.length+1;</span><br><span class="line"> }</span><br><span class="line"> void swap(int[] nums,int i,int j){</span><br><span class="line"> int temp = 0;</span><br><span class="line"> temp = nums[i];</span><br><span class="line"> nums[i] = nums[j];</span><br><span class="line"> nums[j] = temp;</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>#209最短子数组</p>
<ol>
<li>使用left和i两个指针控制</li>
<li>i不断向右延伸，当满足条件时，记录下当前的长度，然后试着从左边缩减<br>本题关键：连续、最短 (适合双指针滑动窗口)<br>不重复（适合Hashset）<br>Java获取int类最大值的方式：Integer.MAX_VALUE 即2^31-1<br>Java获取int类最小值的方式：Integer.MIN_VALUE 即-2^31<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">public int minSubArrayLen(int s, int[] nums) {</span><br><span class="line"> int sum = 0;</span><br><span class="line"> int left = 0;</span><br><span class="line"> int ans = Integer.MAX_VALUE;</span><br><span class="line"> <span class="keyword">for</span>(int i=0;i<nums.length;i++){</span><br><span class="line"> sum+=nums[i];</span><br><span class="line"> <span class="keyword">while</span>(sum>=s){</span><br><span class="line"> ans = Math.min(ans,i-left+1);</span><br><span class="line"> sum -= nums[left];</span><br><span class="line"> left++;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span>(ans == Integer.MAX_VALUE){</span><br><span class="line"> <span class="built_in">return</span> 0;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span>{</span><br><span class="line"> <span class="built_in">return</span> ans;</span><br><span class="line"> }</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>#用两个栈来模拟队列</p>
<ol>
<li>插入时向stack1中插入</li>
<li>删除时从先判断一下stack是否为空</li>
<li>若stack2为空，则将stack1中的元素弹出再顺序入stack2</li>
<li>若stack2不为空，则直接将stack2的栈顶弹出<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line">class CQueue {</span><br><span class="line"></span><br><span class="line"> private Stack<Integer> stack1 = new Stack<Integer>();</span><br><span class="line"> private Stack<Integer> stack2 = new Stack<Integer>();</span><br><span class="line"> public <span class="function"><span class="title">CQueue</span></span>() {</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> public void appendTail(int value) {</span><br><span class="line"> stack1.push(value);</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> public int <span class="function"><span class="title">deleteHead</span></span>() {</span><br><span class="line"> <span class="keyword">if</span>(stack1.empty() && stack2.empty()){</span><br><span class="line"> <span class="built_in">return</span> -1;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span>(stack2.empty()){</span><br><span class="line"> <span class="keyword">while</span>(!stack1.empty()){</span><br><span class="line"> stack2.push(stack1.pop());</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">return</span> stack2.pop();</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>Java Stack<br><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">创建：Stack<Integer> stack = new Stack<Integer>();</span><br><span class="line">empty()、pop()、peek()、push()</span><br></pre></td></tr></table></figure></p>
<p>#求二维数组的连通子图</p>
<ol>
<li>创建一个足够大的数组</li>
<li>使用一部分的空间填写数据</li>
<li>不要顶格写，防止越界</li>
<li>从map[i][j]==1处开始，不断向上下左右四个方向递归调用，dfs函数的参数是当前这一步的index，在递归调用时注意将原本的格子置为0<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br></pre></td><td class="code"><pre><span class="line">public class PoolNum {</span><br><span class="line"></span><br><span class="line"> static int[][] map = new int[100][100];</span><br><span class="line"> //深度优先搜索</span><br><span class="line"> static void dfs(int i,int j){</span><br><span class="line"> <span class="keyword">if</span>(map[i+1][j]==1){</span><br><span class="line"> map[i+1][j]=0;</span><br><span class="line"> dfs(i+1,j);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span>(map[i][j+1]==1){</span><br><span class="line"> map[i][j+1]=0;</span><br><span class="line"> dfs(i,j+1);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span>(map[i-1][j]==1){</span><br><span class="line"> map[i-1][j]=0;</span><br><span class="line"> dfs(i-1,j);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span>(map[i][j-1]==1){</span><br><span class="line"> map[i][j-1]=0;</span><br><span class="line"> dfs(i,j-1);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> public static void main(String[] args) {</span><br><span class="line"> Scanner scanner = new Scanner(System.in);</span><br><span class="line"> int t = scanner.nextInt();</span><br><span class="line"> int count = 0;</span><br><span class="line"> <span class="keyword">for</span>(int i=0;i<t;i++){</span><br><span class="line"> //行数</span><br><span class="line"> int m = scanner.nextInt();</span><br><span class="line"> //列数</span><br><span class="line"> int n = scanner.nextInt();</span><br><span class="line"> <span class="keyword">for</span>(int j = 1;j<m+1;j++){</span><br><span class="line"> <span class="keyword">for</span>(int k=1;k<n+1;k++){</span><br><span class="line"> map[j][k]=scanner.nextInt();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">for</span>(int j = 1;j<m+1;j++){</span><br><span class="line"> <span class="keyword">for</span>(int k=1;k<n+1;k++){</span><br><span class="line"> <span class="keyword">if</span>(map[j][k]==1){</span><br><span class="line"> count++;</span><br><span class="line"> map[j][k]=0;</span><br><span class="line"> dfs(j,k);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> System.out.println(count);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>创建hashMap的方式<br><figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Map<K,V> map = new HashMap<K,V>();</span><br></pre></td></tr></table></figure></p>
<p>hashMap的put既管更新也管添加<br>遍历打印HashMap的方式<br>for(K key:map.keySet()){<br> System.out.println(key+” “+map.get(key));<br>}</p>
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<a class="article-title" href="/2020/06/26/Android开发屏幕显示/">Android开发屏幕显示</a>
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<p>#像素<br>常用的像素单位：px、dp、sp</p>
<ol>
<li>px与物理设备的像素密度有关，同样的屏幕尺寸，像素密度越高，屏幕越清晰</li>
<li>dp与物理设备无关，只与屏幕有关。当density=1时，dp与px是1比1关系</li>
<li>sp与dp类似，专门用于字体大小。sp设置的字体大小可以随系统调整</li>
<li>在xml文件中使用的像素单位是dp、sp</li>
<li>有的函数的参数使用的单位是px，可以用density变量实现px和dp的互换</li>
<li>通过DisplayMetrics对象可以获得屏幕的宽度和高度，以及像素密度</li>
</ol>
<p>#颜色</p>
<ol>
<li>可以使用6位16进制数或者8位16进制数（带透明度）两种方式。代码中0x开头，xml文件中#开头。值越高越亮，值越低越暗。</li>
<li>在Colors.xml中定义颜色，代码中R.color.引用；xml文件使用@color/引用</li>
<li>FF的alpha值表示完全不透明</li>
</ol>
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<time datetime="2020-06-26T01:54:28.000Z" itemprop="datePublished">2020-06-26</time>
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<p>#树的递归定义</p>
<ol>
<li>有且只有一个结点没有父结点</li>
<li>除根以外，其它结点有且仅有一个父结点</li>
<li>每个结点都构成一个以它为根的子树</li>
</ol>
<p>ps:前中后指的是根的位置<br>前中后搜索都属于深度优先搜索</p>
<p>#112判断路经总和</p>
<ol>
<li>采用前序遍历的方式</li>
<li>前序遍历的操作部分改成判断叶子节点是否等于最后相减的值</li>
<li>参数除了树节点外还有一个sum值，雁过拔毛法</li>
<li>在使用树节点遍历时，判断结点为空的处理是必须要有的</li>
<li>Java中虽然没有引用，但是可以用属性（全局变量）来实现相同的效果<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line">class Solution {</span><br><span class="line"> boolean ans = <span class="literal">false</span>;</span><br><span class="line"> public boolean hasPathSum(TreeNode root, int sum) {</span><br><span class="line"> <span class="keyword">if</span>(root == null){</span><br><span class="line"> <span class="built_in">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> preorder(root,sum);</span><br><span class="line"> <span class="built_in">return</span> ans;</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> void preorder(TreeNode root,int sum){</span><br><span class="line"> <span class="keyword">if</span>(root == null){</span><br><span class="line"> <span class="built_in">return</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span>(root.left==null && root.right==null && root.val == sum){</span><br><span class="line"> ans = <span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> preorder(root.left,sum-root.val);</span><br><span class="line"> preorder(root.right,sum-root.val);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>#113 判断路经总和升级版<br>需要返回的不是true或者false，而是返回一个结果的集合（使用List实现）</p>
<ol>
<li>先入栈，加值</li>
<li><p>叶子节点判断条件，满足则加入最终的结果集，注意要创建一个副本去加入，否则会是空集</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">List<Integer> temp = new ArrayList<>(path);</span><br></pre></td></tr></table></figure>
</li>
<li><p>再出栈，减值</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line">class Solution {</span><br><span class="line"> int currSum = 0;</span><br><span class="line"> public List<List<Integer>> pathSum(TreeNode root, int sum) {</span><br><span class="line"> //存储结果集</span><br><span class="line"> List<List<Integer>> res = new ArrayList<>();</span><br><span class="line"> //存储单一结果</span><br><span class="line"> List<Integer> path = new ArrayList<>();</span><br><span class="line"> <span class="keyword">if</span>(root == null){</span><br><span class="line"> <span class="built_in">return</span> res;</span><br><span class="line"> }</span><br><span class="line"> preorder(root,sum,path,res);</span><br><span class="line"> <span class="built_in">return</span> res;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> void preorder(TreeNode root,int sum,List<Integer> path,List<List<Integer>> res){</span><br><span class="line"> <span class="keyword">if</span>(root == null){</span><br><span class="line"> <span class="built_in">return</span>;</span><br><span class="line"> }</span><br><span class="line"> currSum += root.val;</span><br><span class="line"> path.add(root.val);</span><br><span class="line"> <span class="keyword">if</span>(root.left==null && root.right==null && currSum == sum ){</span><br><span class="line"> List<Integer> temp = new ArrayList<>(path);</span><br><span class="line"> res.add(temp);</span><br><span class="line"> }</span><br><span class="line"> preorder(root.left,sum,path,res);</span><br><span class="line"> preorder(root.right,sum,path,res);</span><br><span class="line"> </span><br><span class="line"> currSum -= root.val;</span><br><span class="line"> path.remove(path.size()-1);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</li>
</ol>
<p>ps:ArrayList的部分操作,这几个也是collections体系种类库的共有操作</p>
<ol>
<li>add从后向前加入</li>
<li>get得到指定索引元素</li>
<li>set替换指定索引元素</li>
<li>remove可以按元素删除，也可以按索引删除</li>
</ol>
<p>#树的最近公共祖先</p>
<ol>
<li>找到从根到两个结点的两条路径</li>
<li>将两条路径的结点存储到List中</li>
<li>从后向前比较第一个相同的结点</li>
<li>在深度优先搜索的基础上，前序部分操作为入栈，比较返回；后序部分为出栈</li>
<li>深度搜索基本框架：判断空树->前序操作（入栈）->满足条件->递归调用->后续操作（出栈）</li>
<li>new ArrayList<>(res)可以创建一个结果的副本<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br></pre></td><td class="code"><pre><span class="line">class Solution {</span><br><span class="line"> List<TreeNode> path = new ArrayList<>();</span><br><span class="line"> List<TreeNode> res;</span><br><span class="line"> boolean isFinished;</span><br><span class="line"> public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {</span><br><span class="line"> <span class="keyword">if</span>(root == null){</span><br><span class="line"> <span class="built_in">return</span> null;</span><br><span class="line"> }</span><br><span class="line"> preorder(root,p);</span><br><span class="line"> isFinished = <span class="literal">false</span>;</span><br><span class="line"> List<TreeNode> list1 = new ArrayList<>(res);</span><br><span class="line"> preorder(root,q);</span><br><span class="line"> List<TreeNode> list2 = new ArrayList<>(res);</span><br><span class="line"> <span class="keyword">for</span>(int i = Math.min(list1.size(),list2.size())-1;i>=0;i--){</span><br><span class="line"> <span class="keyword">if</span>(list1.get(i) == list2.get(i)){</span><br><span class="line"> <span class="built_in">return</span> list1.get(i);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="built_in">return</span> null; </span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> void preorder(TreeNode root,TreeNode search){</span><br><span class="line"> //空树判断+结束条件</span><br><span class="line"> <span class="keyword">if</span>(root == null||isFinished == <span class="literal">true</span>){</span><br><span class="line"> <span class="built_in">return</span>;</span><br><span class="line"> }</span><br><span class="line"> //前序操作</span><br><span class="line"> path.add(root);</span><br><span class="line"> <span class="keyword">if</span>(root == search){</span><br><span class="line"> isFinished = <span class="literal">true</span>;</span><br><span class="line"> res = new ArrayList<>(path);</span><br><span class="line"> }</span><br><span class="line"> //递归左右子树</span><br><span class="line"> preorder(root.left,search);</span><br><span class="line"> preorder(root.right,search);</span><br><span class="line"> //后续操作</span><br><span class="line"> path.remove(path.size()-1);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure></li>
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