leetcode合并两个排序链表

题目描述：
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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

/**
 * 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(NULL == l1)
        {
            return l2;
        }
        if(NULL == l2)
        {
            return l1;
        }
        
        if(l1->val > l2->val)
        {
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        }
        else
        {
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        }
    }
};