题解 | #删除链表的倒数第n个节点# 双指针

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @param n int整型 
     * @return ListNode类
     */
    public ListNode removeNthFromEnd (ListNode head, int n) {
        // write code here
        // 找到倒数第n + 1个节点
        if (head == null) {
            return null;
        }
        ListNode hair = new ListNode(0);
        hair.next = head;
        ListNode begin = hair;
        ListNode end = hair;
        int k = 1;
        while (end.next != null) {
            if (k < n + 1) {
                end = end.next;
                k++;
                continue;
            }
            begin = begin.next;
            end = end.next;
        }

        if (k < n + 1) {
            return head;
        }
        // 删除节点begin.next
        begin.next = begin.next.next;
        return hair.next;
    }
}