21.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 * int val;
 * ListNode next;
 * ListNode() {}
 * ListNode(int val) { this.val = val; }
 * ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
// 递归法
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if(list1 == null)
            return list2;
        if(list2==null)
            return list1;
        if(list1.val<list2.val){
            list1.next = mergeTwoLists(list1.next,list2);
            return list1;
        }
        list2.next = mergeTwoLists(list1,list2.next);
        return list2;

    }
}
// 迭代法
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode();
        ListNode cur = dummy;
        while(list1!=null&&list2!=null){
            if(list1.val<list2.val){
                cur.next = list1;
                list1 = list1.next;
            }
            else{
                cur.next = list2;
                list2 = list2.next;
            }
            cur = cur.next;
        }
        cur.next = list1!=null?list1:list2;
        return dummy.next;

    }
}

// 第三次做法
/**
 * Definition for singly-linked list.
 * public class ListNode {
 * int val;
 * ListNode next;
 * ListNode() {}
 * ListNode(int val) { this.val = val; }
 * ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode();
        ListNode cur = dummy;

        while(list1!=null&&list2!=null){
            if(list1.val<list2.val){
                cur.next = list1;
                cur = cur.next;
                list1 = list1.next;
            }
            else{
                cur.next = list2;
                cur = cur.next;
                list2 = list2.next;
            }
        }
        cur.next = list1==null?list2:list1;
        return dummy.next;


    }
}