#24 Swap Nodes in Pairs

<span style="font-family: Arial, Helvetica, sans-serif; background-color: rgb(255, 255, 255);">題目鏈接：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;
}