21. Merge Two Sorted Lists(C++)

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.

Solution 

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