[LeetCode OJ]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.

/**
* 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)
return head;

ListNode* p1 = head;
ListNode* p2 = head->next;
ListNode* temp = NULL;

if(p2 != NULL)
head = p2;
else
return head;

ListNode* last = NULL;

while(p1 != NULL && p2 != NULL)
{
if(last != NULL)
last->next = p2;

temp = p2->next;
p1->next = temp;
p2->next = p1;

last = p1;
p1 = temp;

if(p1 != NULL)
p2 = p1->next;
else
break;
}
return head;
}
};

开始的时候没有考虑last这个指针变量，于是第二组之后跟前面的就连不上了。。。

觉得这道题也还不错。