[LeetCode] 24. Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     struct ListNode *next;
 6  * };
 7  */
 8 struct ListNode* swapPairs(struct ListNode* head) {
 9     struct ListNode* p1;
10     struct ListNode* p2;
11     struct ListNode* tmp;
12     if (!head || !(head->next)){
13         return head;
14     }
15     
16     p1 = head;
17     p2 = p1->next;
18     
19     tmp = p2->next;
20     head = p2;
21     p2->next = p1;
22     p1->next = swapPairs(tmp);
23     
24     return head;
25 }