[leetcode 26] Swap Nodes in Pairs

题目：

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.

思路：

1.题目比较简单，一个一个换就可以，第一次交换需要更换一下头节点；

代码：

class Solution{
public:
	ListNode *swapPairs(ListNode *head)
	{
		if(!head)	return NULL;
		ListNode *first=head,*second=head,*pre=head;
		int flag=0;
		while(first->next)
		{
			second=first->next;
			first->next=second->next;
			second->next=first;
			if(!flag)
			{
				head=second;
				++flag;
			}
			else
				pre->next=second;
			pre=first;
			if(first->next)
				first=first->next;
		}
		return head;
	}
};