合并有序数组&合并有序链表
双指针

/**
 * 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) {
        if(list1==null){return list2;}
        if(list2==null){return list1;}

        ListNode list3 = new ListNode(0);
        ListNode p = list3;
        while(list1!=null && list2!=null){
            if(list1.val>list2.val){
                p.next = list2;
                list2 = list2.next;
            }else{
                p.next = list1;
                list1 = list1.next;
            }
            p = p.next;
        }
        if(list1==null){
            //p=list2;
            p.next = list2;
        }
        if(list2==null){
            //p = list1;
            p.next = list1;
        }
        //return list3;
        return list3.next;
    }
}

1. 最后执行的输入：[1,2,4][1,3,4]
   return list3; 输出：[0,1,1,2,3,4,4]
2. p=list2;p当前指向节点消失