/**

 * 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 head(INT_MIN);//

        ListNode* out = &head;

        while(l1&&l2)//

        {

            if(l1->val < l2->val) { out->next = l1; l1 = l1->next;}

            else {out->next = l2; l2 = l2->next; }

            out = out->next;//

        }

        out->next = l1 ? l1 : l2;//

        return head.next;//

    }

};