#24 Swap Nodes in Pairs

<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">题目链接：https://leetcode.com/problems/swap-nodes-in-pairs</span>

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.

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     struct ListNode *next;
* };
*/
struct ListNode* swapPairs(struct ListNode* head) {
struct ListNode *preNode, *firstNode, *secondNode;
if(head == NULL || head->next == NULL)
return head;
firstNode = head;
secondNode = head->next;
head = secondNode;
firstNode->next = secondNode->next;
head->next = firstNode;
preNode = firstNode;
while(preNode->next != NULL && preNode->next->next != NULL) {
firstNode = preNode->next;
secondNode = firstNode->next;
preNode->next = secondNode;
firstNode->next = secondNode->next;
secondNode->next = firstNode;
preNode = firstNode;
}
return head;
}