LeetCode 21. Merge Two Sorted Lists（合并两个有序链表）

题意：合并两个有序链表

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