No.21 Merge Two Sorted List

 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     {
13         if(l1 == NULL)
14             return l2;
15         if(l2 == NULL)
16             return l1;
17         
18         ListNode *head, *current;
19         //使用头结点的方法：ListNode helper = new ListNode(0); helper.next = head1;
20         //他们都用的是将l2的节点插入l1中的方法
21 
22         if(l1->val <= l2->val)
23         {    
24             head = l1;
25             l1 = l1->next;
26         }
27         else
28         {
29             head = l2;
30             l2 = l2->next;
31         }
32         
33         current = head;
34         
35         while(l1 && l2)
36         {
37             if(l1->val <= l2->val)
38             {    
39               current->next = l1;
40               l1 = l1->next;
41             }
42             else
43              {    
44               current->next = l2;
45               l2 = l2->next;
46             }
47             current = current->next;
48         }
49         
50         if(l1)
51             current->next = l1;
52         if(l2)
53             current->next = l2;
54             
55         return head;
56     }
57 };