<?php
/**
 * https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/
 * 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 ListNode {
    public $val = 0;
    public $next = null;
    function __construct($val = 0, $next = null) {
        $this->val = $val;
        $this->next = $next;
    }
    private function tail() {
        for ($p = $this; $p->next != null; $p = $p->next) {}
        return $p;
    }
    public function add($val) {
        $this->tail()->next = new ListNode($val);
        return $this;
    }
    public function __toString() {
        $s = $this->val;
        for ($p = $this->next; $p != null; $p = $p->next) {
            $s .= ",".$p->val;
        }
        return $s;
    }
}

class Solution {

    /**
     * @param ListNode $head
     * @param Integer $n
     * @return ListNode
     */
    function removeNthFromEnd($head, $n) {
        for ($p = $head, $i = 0; $i < $n && $p != null;
             $i++, $p = $p->next) {}
        // printf("i=%d, n=%d\n", $i, $n);
        // var_dump($p);
        if ($p == null) {
            if ($i < $n) {
                return $head;
            }
            if ($i == $n) {
                $toDel = $head;
                $head = $head->next;
                unset($toDel);
                return $head;
            }
        }
        // prev node to delete
        for ($q = $head; $p->next != null; 
            $p = $p->next, $q = $q->next) {}
        $toDel = $q->next;
        // delete q
        $q->next = $toDel->next;
        unset($toDel);
        return $head;
    }
}


$list = new ListNode(1);
$list->add(2)->add(3)->add(4)->add(5);
echo $list.PHP_EOL;
$s = new Solution();
$list = $s->removeNthFromEnd($list, 3);
echo $list.PHP_EOL;
$list = $s->removeNthFromEnd($list, 4);
echo $list.PHP_EOL;

$ php DeleteNode.php
1,2,3,4,5
1,2,4,5
2,4,5
?

?