21. Merge Two Sorted Lists

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
12         if (l1 == nullptr)
13         {
14             return l2;
15         }
16         else if(l2 == nullptr)
17         {
18             return l1;
19         }
20         
21         ListNode* merge = nullptr;
22         if (l1->val <= l2->val)
23         {
24             merge = l1;
25             merge->next = mergeTwoLists (l1->next, l2);
26         }
27         else
28         {
29             merge = l2;
30             merge->next = mergeTwoLists (l1, l2->next);
31         }
32         
33         return merge;
34     }
35 };