/**
 * 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) {
    auto head=new ListNode(-1),tail=head;
    while(l1&&l2)
    if(l1->val<=l2->val)
    tail=tail->next=l1,l1=l1->next;
    else tail=tail->next=l2,l2=l2->next;
    if(l1)tail->next=l1;
    else tail->next=l2;
    return head->next;
    }
};