<?php

//Definition for a binary tree node.

class TreeNode {

public $val = null;

public $left = null;

public $right = null;

function __construct($value) { $this->val = $value; }

}

//Definition for a singly-linked list.

class ListNode {

public $val = 0;

public $next = null;

function __construct($val) { $this->val = $val; }

}

class Solution {

/**

* @param TreeNode $tree

* @return ListNode[]

*/

function listOfDepth($tree) {

if($tree === null){

return [];

}

$queue = new SplQueue();

$queue->enqueue([$tree, 0]);

$result = [];

$p = null;

while (!$queue->isEmpty()){

$item = $queue->dequeue();

if(isset($result[$item[1]])){

$p->next = new ListNode($item[0]->val);

$p = $p->next;

}else{

$p = new ListNode($item[0]->val);

$result[$item[1]] = $p;

}

if($item[0]->left !== null){

$queue->enqueue([$item[0]->left, $item[1] + 1]);

}

if($item[0]->right !== null){

$queue->enqueue([$item[0]->right, $item[1] + 1]);

}

}

return $result;

}

}