remove nth node from end of list

单链表定义
public class ListNode{
	int val;
	ListNode next;
	ListNode(int x){val = x;}
}

public class Solution{
	public ListNode removeNthNodeFromEnd(ListNode head , int n){
		ListNode start = new ListNode(0);
		start.next = head;
		ListNode fast = start;
		ListNode slow = start;
		
		for(int i = 0 ; i <= n ; i++){	//让fast和slow两指针之间gap为n
			fast = fast.next;
		}
		
		while(fast.next != null){	//fast和slow指针同速前进，直到最后指向null
			fast = fast.next;
			slow = slow.next;
		}
		slow.next = slow.next.next;	//删除一个节点
		return start.next;
	}
}