leetcode | Swap Nodes in Pairs in a linklist

Swap Nodes in Pairs : https://leetcode.com/problems/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.

注：首先画图理清思路，再开始写代码。
p->t->q : 交换 t 和 q，但要知道 p
关键点：必须注意，交换之后，p->q->t，所以新的 p 应该指向 t

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if (head == NULL || head->next == NULL)
            return head;
        ListNode* dummy = new ListNode(-1);
        dummy->next = head;
        ListNode* p = dummy;

        while (p && p->next && p->next->next) {
            ListNode* t = p->next;
            ListNode* q = p->next->next;
            p->next = q;
            t->next = q->next;
            q->next = t;

            p = t; // 注意此时，t已经被交换到q的后面
        }
        return dummy->next;
    }
};