Leetcode -- 21 Merge Two Sorted Lists

递归的思路：

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