155.java

class MinStack {
    Stack<Integer> stack = new Stack<>();
    Stack<Integer> minStack = new Stack<>();

    public MinStack() {

    }

    public void push(int val) {
        stack.push(val);
        if (minStack.isEmpty() || minStack.peek() >= val) {
            minStack.push(val);
        } else {
            minStack.push(minStack.peek());
        }

    }

    public void pop() {
        stack.pop();
        minStack.pop();
    }

    public int top() {
        return stack.peek();
    }

    public int getMin() {
        return minStack.peek();
    }
}

// 第二次做法
/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */


 class MinStack {

    // 每次寸一个数组，0号位置表示当前元素，1号位置表示最小元素
    private final Deque<int[]> stack = new ArrayDeque<>();

    public MinStack() {
        stack.push(new int[]{0,Integer.MAX_VALUE});
    }
    
    public void push(int val) {
        stack.push(new int[]{val,Math.min(getMin(),val)});
    }
    
    public void pop() {
        stack.pop();
    }
    
    public int top() {
        return stack.peek()[0];
    }
    
    public int getMin() {
        return stack.peek()[1];
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */