[leetcode 21] Remove Nth Node From End of List

题目：

Given a linked list, remove the nth node from the end of list and return its head.

For example,

   Given linked list: 1->2->3->4->5, and n = 2.   After removing the second node from the end, the linked list becomes 1->2->3->5.

Note:
Given n will always be valid.
Try to do this in one pass.

思路：

1.快慢指针

代码：

class Solution{public:ListNode *removeNthFromEnd(ListNode *head,int n){ListNode *slow,*fast,*pre;slow=fast=pre=head;for(int i=0;i!=n;++i)fast=fast->next;while(fast){fast=fast->next;pre=slow;slow=slow->next;}if(slow==head)    head=head->next;elsepre->next=slow->next;return head;}};