leetcode 21

合并两个有序数列。属于基础的数据结构问题，核心在于对链表的操作。
代码如下：
 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         ListNode* head1 = NULL;
13         ListNode* head2 = NULL;
14         ListNode* tmp = NULL;
15         if(l1 == NULL)
16         {
17             return l2;
18         }
19         else if(l2 == NULL)
20         {
21             return l1;
22         }
23         else if(l1->val > l2->val)
24         {
25             tmp = l2;
26             head1 = l1;
27             head2 = l2->next;
28         }
29         else
30         {
31             tmp = l1;
32             head1 = l1->next;
33             head2 = l2;
34         }
35         ListNode* node = tmp;
36         while(true)
37         {
38             if(head1 == NULL)
39             {
40                 node->next = head2;
41                 break;
42             }
43             if(head2 == NULL)
44             {
45                 node->next = head1;
46                 break;
47             }
48             if(head1->val > head2->val)
49             {
50                 node->next = head2;
51                 head2 = head2->next;
52             }
53             else
54             {
55                 node->next = head1;
56                 head1 = head1->next;
57             }
58             node = node->next;
59         }
60         return tmp;
61     }
62 };