Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

/**
 * 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) {
    ListNode dummy(INT_MIN);
    ListNode *tail = &dummy;
    while(l1&&l2){
       if(l1->val<l2->val){
        tail->next = l1;
        l1 = l1->next;
        }else{
        tail->next = l2;
        l2 = l2->next;
      } 
        tail = tail->next;
    }
    tail->next = l1?l1:l2;
    return dummy.next;
  }
};