題目描述：

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

例子：

   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.

分析：
    題意：給定一個單鏈表，移除它的倒數第n個結點並返回。

    思路：為了方便操作，我們用指標h建立一個空結點（方便指標

程式碼：

#include <bits/stdc++.h>

using namespace std;

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

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
		// Exceptional Case: 
		if(!head){
			return head;
		}
        ListNode* h = new ListNode(-1), *pre = h, *p = head, *q = head;
		h->next = head;
		for(int i = 1; i <= n - 1; i++){
			q = q->next;
		}
		while(q->next){
			q = q->next;
			pre = pre->next;
			p = p->next;
		}
		pre->next = p->next;
		delete(p);
		return h->next;
    }
};