leetcode 21 list merge

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
         if(l1==NULL)return l2;
         if(l2==NULL)return l1;
         ListNode *l3,*head;
         if(l1->val<l2->val) {
             l3=l1;
             l1=l1->next;
         }
         else {
             l3=l2;
             l2=l2->next;
         }
         head=l3;
         while(l1!=NULL&&l2!=NULL) {
             if(l1->val<=l2->val) {
                 l3->next=l1;
                 l1=l1->next;
                 l3=l3->next;
             }
             else {
                 l3->next=l2;
                 l2=l2->next;
                 l3=l3->next;
             }
         }
         while(l1!=NULL) {
             l3->next=l1;
             l1=l1->next;
             l3=l3->next;
         }
          while(l2!=NULL) {
             l3->next=l2;
             l2=l2->next;
             l3=l3->next;
         }
         return head;
     }
 };