[LeetCode]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) return l2;
13         if (!l2) return l1;
14         
15         ListNode dummy(-1);
16         ListNode *p = &dummy;
17         while (l1 != nullptr && l2 != nullptr) {
18             if (l1->val < l2->val) {
19                 p->next = l1;
20                 l1 = l1->next;
21             } else {
22                 p->next = l2;
23                 l2 = l2->next;
24             }
25             p = p->next;
26         }
27         
28         if (!l1) {
29             p->next = l2;
30         } else {
31             p->next = l1;
32         }
33         
34         return dummy.next;
35     }
36 };