Leetcode 21. Merge Two Sorted Lists

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
12         if(!l1) return l2;
13         if(!l2) return l1;
14         if(l1->val<l2->val){
15             l1->next=mergeTwoLists(l1->next,l2);
16             return l1;
17         }
18         l2->next=mergeTwoLists(l1,l2->next);
19         return l2;
20     }
21 };

 1 class Solution {
 2 public:
 3     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
 4         ListNode* head=new ListNode(-1);
 5         ListNode* end=head;
 6         while(l1&&l2){
 7             if(l1->val<l2->val){
 8                 end->next=l1;
 9                 l1=l1->next;
10             }
11             else{
12                 end->next=l2;
13                 l2=l2->next;
14             }
15             end=end->next;
16         }
17         if(l1){
18             end->next=l1;
19         }
20         else{
21             end->next=l2;
22         }
23         return head->next;
24     }
25 };