/**

  * 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;

         if(l1->val > l2->val)

             return mergeTwoLists(l2, l1);

         ListNode* head = l1;

         while(l1->next && l2){

             if(l1->next->val > l2->val){

                 ListNode* next = l2->next;

                 l2->next = l1->next;

                 l1->next = l2;

                 l2 = next;

             }

             l1 = l1->next;

         }

         if(l2) l1->next = l2;

         return head;

     }

 };