/
* 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(l2&&l1)
{
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;
}

};