/**
 * 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) return l2;
        if(!l2) return l1;
        
          ListNode *res = new ListNode(0);
        ListNode *tmp = res;
        while(l1 && l2){
            if(l1->val <= l2->val){
                tmp->next = l1;
                tmp = l1;
                l1 = l1->next;
            }
            else{
                tmp->next = l2;
                tmp = l2;
                l2 = l2->next;
            }
        }
        if(l1)
            tmp->next = l1;
        if(l2)
            tmp->next = l2;
        
        return res->next;
    }
};