/**
* 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 dummy = new ListNode(-1), tail = dummy;
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;
if(l2) tail->next = l2;
return dummy->next;
}
};