Solution to a problem to check if a binary tree is balanced

binary_tree_balanced.php

<?php


// check if a binary tree is balanced (it means | max-depth - min-depth | <= 1) as an AVL tree

// time complexity is O(N) , where N is the number of nodes
// space complexity iis O(h) , where h is the number of levels


class Node {

    public $data;
    public $left;
    public $right;

    public function __construct($data)
    {
        $this->data = $data;
        $this->left = $this->right = NULL;
    }

    public function getData() {
        return $this->data;
    }

}

class BinaryTree {

    private $root;
    private $nodes;

    public function __construct($droot,$adiacence_list)
    {
        $this->nodes = array();
        $this->root = NULL;

        foreach($adiacence_list as $root => $leafs) {
            if(!isset($this->nodes[$leafs[0]])) {
                $left = new Node($leafs[0]);
                $this->nodes[$leafs[0]] = $left;
            }
            if(!isset($this->nodes[$leafs[1]])) {
                $right = new Node($leafs[1]);
                $this->nodes[$leafs[1]] = $right;
            }

            $node = new Node($root);
            $node->left = &$this->nodes[$leafs[0]];
            $node->right = &$this->nodes[$leafs[1]];
            $this->nodes[$root] = $node;


        }
        $this->root = &$this->nodes[$droot];
    }

    public function listNodes() {
        var_dump($this->root);
    }

    public function maxDepth($root) {

        if($root == NULL) {
            return 0;
        }
        return 1 + max($this->maxDepth($root->left), $this->maxDepth($root->right));

    }

    public function minDepth($root) {

        if($root == NULL) {
            return 0;
        }
        return 1 + min($this->minDepth($root->left), $this->minDepth($root->right));

    }

    public function isBalanced() {

        return $this->maxDepth($this->root) - $this->minDepth($this->root) <= 1;

    }


}


$ad_list = array(
    'a' => array('b','c'),
    'b' => array('d','e'),
    'd' => array('g')

);

$bt = new BinaryTree('a',$ad_list);

$bt->listNodes();

echo $bt->isBalanced();