Swap Nodes in Pairs

 1 class Solution
 2 {
 3 public:
 4   ListNode *swapPairs(ListNode *head)
 5   {
 6     if(head == NULL) return NULL;
 7     ListNode *cur = head->next, *pre = head, *temp = NULL;
 8     int i = 0;
 9     while(cur)
10     {
11       if(i % 2 == 0)
12       {
13         pre->next = cur->next;
14         cur->next = pre;
15         if(i == 0)
16         {
17           head = cur;
18           temp = head;
19         }
20         else
21         {
22           temp->next = cur;
23           temp = temp->next;
24         }
25         cur = pre->next;
26       }
27       else
28       {
29         temp = temp->next;
30         pre = pre->next;
31         cur = cur->next;
32       }
33       i++;
34     }
35     return head;
36   }
37 };