24.LeetCode Swap Nodes in Pairs(easy)[单链表]

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.

解题思路：要求是不能申请其他的空间，并且不能直接通过改值来完成相邻节点交换。因此只能通过两个指针p和q来不停保存当前相邻节点交换之后的状态。

/**
* 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* nhead = head->next;
ListNode* p = head;
ListNode* q = head;
while(p != NULL && p->next != NULL)
{
ListNode* temp = p->next->next;
ListNode* temp1 = p->next;
p->next->next = p;
q->next = temp1;
p->next = temp;
q = p;
p = temp;
}
return nhead;
}
};