题解 | #删除链表的倒数第n个节点#

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) {
        // 快慢指针解法
        if (head.next == null){
            return head.next;
        }
        ListNode slow = head;
        ListNode fast = head;
        // 领先n步
        for (int i = 0; i < n; i++) {
            fast = fast .next;
        }
        if (fast == null){
            return slow.next;
        }
        // 第n步就是慢指针
        ListNode pre = slow;
        while (fast != null){
            pre = slow;
            slow = slow.next;
            fast = fast.next;
            if (fast == null){
                pre.next = slow.next;
                return head;
            }
        }
        return null;
    }
}