问题描述：

java代码：

/*** 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 res = new ListNode(0);ListNode cur = res;while(list1 != null || list2 != null){if(list1 != null && list2 != null){if (list1.val == list2.val){cur.next = new ListNode(list1.val);cur = cur.next;cur.next = new ListNode(list2.val);list1 = list1.next;list2 = list2.next;}else{if(list1.val>list2.val){cur.next = new ListNode(list2.val);list2 = list2.next;}else{cur.next = new ListNode(list1.val);list1 = list1.next;}}}else if(list1 != null){cur.next = new ListNode(list1.val);list1 = list1.next;}else if(list2 != null){cur.next = new ListNode(list2.val);list2 = list2.next;}cur = cur.next;}return res.next;}
}