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         ListNode* newHead = new ListNode(-1);
13         ListNode* tail = newHead;
14         
15         while(l1 != NULL && l2 != NULL)
16         {
17             if(l1->val <= l2->val){
18                 tail->next = l1;
19                 l1 = l1->next;
20             }
21             else{
22                 tail->next = l2;
23                 l2 = l2->next;
24             }
25             tail =tail->next;
26         }
27         if(l1 != NULL){
28             tail->next = l1;
29         }else if(l2 != NULL){
30             tail->next = l2;
31         }
32         return newHead->next;
33     }
34 };