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) {
        if (head == null) {
            return null;
        }
        //两次遍历就可以解决
        //统计节点数量
        int count = 0;
        ListNode node = head;
        while (node != null) {
            count++;
            node = node.next;
        }
        int index = 0;
        int tag = count - n + 1;
        if (tag < 0) {
            return null;
        }
        ListNode currentNode = head;
        ListNode preNode = null;
        while (currentNode != null) {
            index++;
            if (index == tag) {
                if (preNode != null) {
                    preNode.next = currentNode.next;
                    return head;
                } else {
                    return head.next;
                }
            }
            preNode = currentNode;
            currentNode = currentNode.next;
        }
        return null;
    }
}