leetcode[21]Merge Two Sorted 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) 
    {
        if(l1==NULL)return l2;
        if(l2==NULL)return l1;
        ListNode *res;
        if(l1->val<l2->val)
        {
            res=l1;
            l1=l1->next;
            res->next=mergeTwoLists(l1,l2);
        }
        else
        {
            res=l2;
            l2=l2->next;
            res->next=mergeTwoLists(l1,l2);
        }
        return res;
    }
};