PHP Binary Tree Recursive Traversal Infinite Loop Issue

Question

I have a binary tree and node class that can create nodes and then recursively traverse the root for pre, post and in-order node orders. This code works when in JS, but for some reason infinitely loops with a warning of "Cannot use '$this' in non-object context." when returning $this in the addSide() function. What is causing this infinite loop, and how can I fix it?

<?php

class Node {
      public $value;
      public $right = null;
      public $left = null;

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

    class BinaryTree {
      public $root;

      function __constructor() {}

      function create($value) {
        $newNode = new Node($value);
        if (!$this->root) {
          $this->root = $newNode;
          return $this; //no warning
        }
        $current = $this->root;

        function addSide($side, $current, $newNode) {
          if (!$current->$side) {
            $current->$side = $newNode;
            return $this; //Warning: "Cannot use '$this' in non-object context."
          }
          $current = $current->$side;
        };
        while (true) {
          if ($value === $current->value) return $this;
          if ($value < $current->value) addSide("left", $current, $newNode);
          else addSide("right", $current, $newNode);
        }
      }

      function preOrder() {
        $visited = [];
        $current = $this->root;

        function traversePreOrder($node) {
          array_push($visited, $node->value);
          if ($node->left) traversePreOrder($node->left);
          if ($node->right) traversePreOrder($node->right);
        };
        traversePreOrder($current);
        return $visited;
      }

      function postOrder() {
        $visited = [];
        $current = $this->root;

        function traversePostOrder($node) {
          if ($node->left) traversePostOrder($node->left);
          if ($node->right) traversePostOrder($node->right);
          array_push($visited, $node->value);
        };

        traversePostOrder($current);
        return $visited;
      }

      function inOrder() {
        $visited = [];
        $current = $this->root;

        function traverseInOrder($node) {
          if ($node->left) traverseInOrder($node->left);
          array_push($visited, $node->value);
          if ($node->right) traverseInOrder($node->right);
        };

        traverseInOrder($current);
        return $visited;
      }
    }

    $tree = new BinaryTree();

    $tree->create(50);
    $tree->create(30);
    $tree->create(45);
    $tree->create(12);
    $tree->create(29);

    echo("inOrder: ". $tree->inOrder());
    echo("preOrder: ". $tree->preOrder());
    echo("postOrder: ". $tree->postOrder());

Answer 1

Since you don't seem to be from a PHP background, here are some of the things to note down:

• It is __construct() and not __constructor(). This served to be a major problem in the code during value comparisons.

• No need to create functions inside functions. This can lead to cannot redeclare function issues when a method is called twice.

• When calling a method from another method inside a class, $this-> is necessary unless the function being called is an inbuilt function in PHP or at least available during code execution.

• You seem to be creating a Binary Search Tree instead of just a Binary Tree.

• Pass $visited by reference when collecting values during traversal.

• You can't print arrays using echo. Use print_r() or use implode() to convert the array to string using a delimiter(say ,) and then print it using echo.

• In create(), you sometimes return a node and sometimes $this. Both are not the same. Former one is an object of the Node class and the latter one is the object of the BinaryTree class.

• In create() method, you simply need to traverse left or right from the current code according to the given value, which can be achieved using a simple while loop.

Corrected Code:

<?php

class Node {
  public $value;
  public $right = null;
  public $left = null;

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

class BinaryTree {
  public $root;

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

  function create($value) {
    $newNode = new Node($value);
    if ($this->root === null) {
      $this->root = $newNode;
      return $newNode; //no warning
    }

    $current = $this->root;
    while($current !== null){        
        if($current->value > $value){
            if($current->left === null){
                $current->left = $newNode;
                break;
            }else{
                $current = $current->left;
            }
        }else if($current->value < $value){
            if($current->right === null){
                $current->right = $newNode;
                break;
            }else{
                $current = $current->right;
            }
        }else{
            throw new \Exception("Node with $value already exists.");
        }
    }
    
    return $newNode;
  }

  function preOrder() {
    $visited = [];
    $current = $this->root;
     $this->traversePreOrder($current,$visited);
    return $visited;
  }

  function traversePreOrder($node,&$visited) {
    array_push($visited, $node->value);
    if ($node->left !== null)  $this->traversePreOrder($node->left,$visited);
    if ($node->right !== null)  $this->traversePreOrder($node->right,$visited);
  }

  function postOrder() {
    $visited = [];
    $current = $this->root;
    $this->traversePostOrder($current,$visited);
    return $visited;
  }

  function traversePostOrder($node,&$visited) {
    if ($node->left !== null)  $this->traversePostOrder($node->left,$visited);
    if ($node->right !== null)  $this->traversePostOrder($node->right,$visited);
    array_push($visited, $node->value);
  }

  function inOrder() {
    $visited = [];
    $current = $this->root;
    $this->traverseInOrder($current,$visited);
    return $visited;
  }

  function traverseInOrder($node,&$visited) {
    if ($node->left != null)  $this->traverseInOrder($node->left,$visited);
    array_push($visited, $node->value);
    if ($node->right !== null)  $this->traverseInOrder($node->right,$visited);
  }
}

$tree = new BinaryTree();

$tree->create(50);
$tree->create(30);
$tree->create(45);
$tree->create(12);
$tree->create(29);

echo "inOrder: ". implode(",",$tree->inOrder()),PHP_EOL;
echo "preOrder: ". implode(",",$tree->preOrder()),PHP_EOL;
echo "postOrder: ". implode(",",$tree->postOrder()),PHP_EOL;

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