Remove Nth Node From End of List

问题：删除距离末尾n个距离的结点
分析：先找出距离末尾n个距离的结点其距离开始的距离多少，然后再删除

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

class Solution {

public:

    int front;

    int dfs(ListNode *head,int n,int step)

    {

        if(head==NULL) return 0;

        int h=dfs(head->next,n,step+1)+1; 

        if(h==n) { front=step;}

        return h;

    }

    ListNode *removeNthFromEnd(ListNode *head, int n) {

        if(head==NULL) return head;

        front=0;

        dfs(head,n,1);

        ListNode *help = new ListNode(0);

        help->next=head;

        int t=1;

        ListNode *ret=help;

        while(1)

        {

            if(t==front) 

            {

                ret->next=head->next;

                delete head;

                break;

            }

            ret=ret->next;

            head=head->next;

            t++;

        }

        return help->next;

    }

};