源题目

https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/

19. 删除链表的倒数第 N 个结点

难度中等1514

给你一个链表，删除链表的倒数第 n 个结点，并且返回链表的头结点。
进阶：你能尝试使用一趟扫描实现吗？

示例 1：
输入：head = [1,2,3,4,5], n = 2 输出：[1,2,3,5]
示例 2：
输入：head = [1], n = 1 输出：[]
示例 3：
输入：head = [1,2], n = 1 输出：[1]

提示：

• 链表中结点的数目为 sz
• 1 <= sz <= 30
• 0 <= Node.val <= 100
• 1 <= n <= sz

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {
    /**
     * @param ListNode $head
     * @param Integer $n
     * @return ListNode
     */
    function removeNthFromEnd($head, $n) {
        if($head == null){
            return $head;
        }
        $newList = new ListNode();
        $newList->next = $head;
        $fast = $slow = $newList;
        for($i=0; $i<$n+1; $i++){
            $fast = $fast->next;
        }
        while($fast != null){
            $fast = $fast->next;
            $slow = $slow->next;
        }
        $slow->next = $slow->next->next;
        return $newList->next;
    }
}