记忆中，两个指针合并即可。 

建立哨兵节点dum

/*** 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(0),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;}
}