21. 合并两个有序链表

 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  //思路很清晰，建立一个虚拟节点
10 class Solution 
11 {
12 public:
13     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) 
14     {
15         ListNode* dummy = new ListNode(-1);
16         ListNode* head = dummy;
17         while(l1 && l2)
18         {
19             if(l1->val < l2->val)
20             {
21                 head->next = l1;
22                 l1 = l1->next;
23             }
24             else
25             {
26                 head->next = l2;
27                 l2 = l2->next;
28             }
29             head = head->next;
30         }
31         head->next = l1 ? l1 : l2;
32         return dummy->next;
33     }
34 };