/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        ListNode* head = new ListNode(-1);
        ListNode* res = head;
        while(l1 != NULL&&l2 != NULL){
            if((l1->val)<(l2->val)){
                head->next = l1;
                l1 = l1->next;
            }else{
                head->next = l2;
                l2 = l2->next;
            }
            head = head->next;
        }
        if(l1 != NULL){
            head->next = l1;
        }else{
            head->next = l2;
        }
        return res->next;
    }
};