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;
}
};