Non-recursive program to delete an entire binary tree

We have discussed recursive implementation to delete an entire binary tree here.
We strongly recommend you to minimize your browser and try this yourself first.
Now how to delete an entire tree without using recursion. This could easily be done with the help of Level Order Tree Traversal. The idea is for each dequeued node from the queue, delete it after queuing its left and right nodes (if any). The solution will work as we are traverse all the nodes of the tree level by level from top to bottom, and before deleting the parent node, we are storing its children into queue that will be deleted later.

C++

/* Non-Recursive Program to delete an entire binary tree. */
#include <bits/stdc++.h>
using namespace std;

// A Binary Tree Node
struct Node
{
    int data;
    struct Node *left, *right;
};

/* Non-recursive function to delete an entire binary tree. */
void _deleteTree(Node *root)
{
    // Base Case
    if (root == NULL)
        return;

    // Create an empty queue for level order traversal
    queue<Node *> q;

    // Do level order traversal starting from root
    q.push(root);
    while (!q.empty())
    {
        Node *node = q.front();
        q.pop();

        if (node->left != NULL)
            q.push(node->left);
        if (node->right != NULL)
            q.push(node->right);

        free(node);
    }
}

/* Deletes a tree and sets the root as NULL */
void deleteTree(Node** node_ref)
{
  _deleteTree(*node_ref);
  *node_ref = NULL;
}

// Utility function to create a new tree Node
Node* newNode(int data)
{
    Node *temp = new Node;
    temp->data = data;
    temp->left = temp->right = NULL;

    return temp;
}

// Driver program to test above functions
int main()
{
    // create a binary tree
    Node *root =  newNode(15);
    root->left = newNode(10);
    root->right = newNode(20);
    root->left->left = newNode(8);
    root->left->right = newNode(12);
    root->right->left = newNode(16);
    root->right->right = newNode(25);

    //delete entire binary tree
    deleteTree(&root);

    return 0;
}

Java

/* Non-recursive program to delete the entire binary tree */
import java.util.*;

// A binary tree node
class Node
{
    int data;
    Node left, right;

    public Node(int data)
    {
        this.data = data;
        left = right = null;
    }
}

class BinaryTree
{
    Node root;

    /* Non-recursive function to delete an entire binary tree. */
    void _deleteTree()
    {
        // Base Case
        if (root == null)
            return;

        // Create an empty queue for level order traversal
        Queue<Node> q = new LinkedList<Node>();

        // Do level order traversal starting from root
        q.add(root);
        while (!q.isEmpty())
        {
            Node node = q.peek();
            q.poll();

            if (node.left != null)
                q.add(node.left);
            if (node.right != null)
                q.add(node.right);
        }
    }

    /* Deletes a tree and sets the root as NULL */
    void deleteTree()
    {
        _deleteTree();
        root = null;
    }

    // Driver program to test above functions
    public static void main(String[] args)
    {
        // create a binary tree
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(15);
        tree.root.left = new Node(10);
        tree.root.right = new Node(20);
        tree.root.left.left = new Node(8);
        tree.root.left.right = new Node(12);
        tree.root.right.left = new Node(16);
        tree.root.right.right = new Node(25);

        // delete entire binary tree
        tree.deleteTree();
    }
}

// This code has been contributed by Mayank Jaiswal(mayank_24)

Python3

# Python program to delete an entire binary tree
# using non-recursive approach

# A binary tree node
class Node:

    # A constructor to create a new node
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None

# Non-recursive function to delete an entrie binary tree
def _deleteTree(root):

    # Base Case
    if root is None:
        return

    # Create a empty queue for level order traversal
    q = []

    # Do level order traversal starting from root
    q.append(root)
    while(len(q)>0):
        node = q.pop(0)

        if node.left is not None:
            q.append(node.left)

        if node.right is not None:
            q.append(node.right)

        node = None
    return node

# Deletes a tree and sets the root as None
def deleteTree(node_ref):
    node_ref = _deleteTree(node_ref)
    return node_ref

# Driver program to test above function

# Create a binary tree
root = Node(15)
root.left = Node(10)
root.right = Node(20)
root.left.left = Node(8)
root.left.right = Node(12)
root.right.left = Node(16)
root.right.right = Node(25)

# delete entire binary tree
root = deleteTree(root)


# This program is contributed by Nikhil Kumar Singh(nickzuck_007)

C#

/* Non-recursive program to delete the entire binary tree */
using System;
using System.Collections.Generic;

// A binary tree node
public class Node
{
    public int data;
    public Node left, right;

    public Node(int data)
    {
        this.data = data;
        left = right = null;
    }
}

public class BinaryTree
{
    Node root;

    /* Non-recursive function to
    delete an entire binary tree. */
    void _deleteTree()
    {
        // Base Case
        if (root == null)
            return;

        // Create an empty queue for level order traversal
        Queue<Node> q = new Queue<Node>();

        // Do level order traversal starting from root
        q.Enqueue(root);
        while (q.Count != 0)
        {
            Node node = q.Peek();
            q.Dequeue();

            if (node.left != null)
                q.Enqueue(node.left);
            if (node.right != null)
                q.Enqueue(node.right);
        }
    }

    /* Deletes a tree and sets the root as NULL */
    void deleteTree()
    {
        _deleteTree();
        root = null;
    }

    // Driver program to test above functions
    public static void Main(String[] args)
    {
        // create a binary tree
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(15);
        tree.root.left = new Node(10);
        tree.root.right = new Node(20);
        tree.root.left.left = new Node(8);
        tree.root.left.right = new Node(12);
        tree.root.right.left = new Node(16);
        tree.root.right.right = new Node(25);

        // delete entire binary tree
        tree.deleteTree();
    }
}

// This code has been contributed by 29AjayKumar

Javascript

<script>
/* Non-recursive program to delete the entire binary tree */

// A binary tree node
class Node
{
    constructor(data)
    {
        this.data = data;
        this.left = this.right = null;
    }
}

let root;

/* Non-recursive function to delete an entire binary tree. */
function _deleteTree()
{
    // Base Case
        if (root == null)
            return;

        // Create an empty queue for level order traversal
        let q = [];

        // Do level order traversal starting from root
        q.push(root);
        while (q.length != 0)
        {
            let node = q.shift();


            if (node.left != null)
                q.push(node.left);
            if (node.right != null)
                q.push(node.right);
        }
}

    /* Deletes a tree and sets the root as NULL */
function deleteTree()
{
    _deleteTree();
        root = null;
}

// Driver program to test above functions
root = new Node(15);
root.left = new Node(10);
root.right = new Node(20);
root.left.left = new Node(8);
root.left.right = new Node(12);
root.right.left = new Node(16);
root.right.right = new Node(25);

// delete entire binary tree
deleteTree();

// This code is contributed by rag2127
</script>

Time Complexity: O(n)

As it is a normal level order traversal and we are visiting every node just once.

Auxiliary Space: O(b)

Here b is the breadth of the tree or the maximum number of elements at any level. The extra space is required to store the elements of a level in the queue.

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