Name: Binary Tree to BST DSA Problem
Uploaded: 2024-05-09T15:38:23Z
Duration: 14 min 27 s

Given a Binary Tree, the task is to convert it to a Binary Search Tree. The conversion must be done in such a way that keeps the original structure of the Binary Tree.

Examples

Input:

Output:

Explanation: The above Binary tree is converted to Binary Search tree by keeping the original structure of Binary Tree.

Try It Yourself

Approach:

The idea to recursively traverse the binary tree and store the nodes in an array. Sort the array, and perform in-order traversal of the tree and update the value of each node to the corresponding value in tree.

Below is the implementation of the above approach:

C++

// C++ Program to convert binary 
// tree to binary search tree.
#include <bits/stdc++.h>
using namespace std;

class Node {
public:
    int data;
    Node* left, *right;
    Node(int x) {
        data = x;
        left = nullptr;
        right = nullptr;
    }
};

// Inorder traversal to store the nodes in a vector
void inorder(Node* root, vector<int>& nodes) {
    if (root == nullptr) {
        return;
    }
    inorder(root->left, nodes);
    nodes.push_back(root->data);
    inorder(root->right, nodes);
}

// Inorder traversal to convert tree
// to BST.
void constructBST(Node* root, vector<int> nodes, int& index) {
    if (root == nullptr) return;
    
    constructBST(root->left, nodes, index);
    
    // Update root value
    root->data = nodes[index++];
    
    constructBST(root->right, nodes, index);
}

// Function to convert a binary tree to a binary search tree
Node* binaryTreeToBST(Node* root) {
    vector<int> nodes;
    inorder(root, nodes);
    
    // sort the nodes
    sort(nodes.begin(), nodes.end());
    
    int index = 0;
    constructBST(root, nodes, index);
    return root;
}

// Function to print the inorder traversal of a binary tree
void printInorder(Node* root) {
    if (root == NULL) {
        return;
    }
    printInorder(root->left);
    cout << root->data << " ";
    printInorder(root->right);
}

int main() {
    
    // Creating the tree
    //         10
    //        /  \
    //       2    7
    //      / \
    //     8   4
    Node* root = new Node(10);   
    root->left = new Node(2);    
    root->right = new Node(7);   
    root->left->left = new Node(8);  
    root->left->right = new Node(4);
    
    Node* ans = binaryTreeToBST(root);
    printInorder(ans);

    return 0;
}

Java

// Java Program to convert binary 
// tree to binary search tree.
import java.util.ArrayList;
import java.util.Collections;

class Node {
    int data;
    Node left, right;

    Node(int x) {
        data = x;
        left = null;
        right = null;
    }
}

class GfG {
    
    // Inorder traversal to store the nodes in a vector
    static void inorder(Node root, ArrayList<Integer> nodes) {
        if (root == null) {
            return;
        }
        inorder(root.left, nodes);
        nodes.add(root.data);
        inorder(root.right, nodes);
    }

    // Inorder traversal to convert tree to BST.
    static void constructBST(Node root, 
                             ArrayList<Integer> nodes, int[] index) {
        if (root == null) return;

        constructBST(root.left, nodes, index);

        // Update root value
        root.data = nodes.get(index[0]);
        index[0]++;

        constructBST(root.right, nodes, index);
    }

    // Function to convert a binary tree to a binary search tree
    static Node binaryTreeToBST(Node root) {
        ArrayList<Integer> nodes = new ArrayList<>();
        inorder(root, nodes);

        // sort the nodes
        Collections.sort(nodes);

        int[] index = {0};
        constructBST(root, nodes, index);
        return root;
    }

    // Function to print the inorder traversal of a binary tree
    static void printInorder(Node root) {
        if (root == null) {
            return;
        }
        printInorder(root.left);
        System.out.print(root.data + " ");
        printInorder(root.right);
    }

    public static void main(String[] args) {
        
        // Creating the tree
        //         10
        //        /  \
        //       2    7
        //      / \
        //     8   4
        Node root = new Node(10);
        root.left = new Node(2);
        root.right = new Node(7);
        root.left.left = new Node(8);
        root.left.right = new Node(4);

        Node ans = binaryTreeToBST(root);
        printInorder(ans);
    }
}

Python

# Python Program to convert binary 
# tree to binary search tree.
class Node:
    def __init__(self, x):
        self.data = x
        self.left = None
        self.right = None

# Inorder traversal to store the nodes in a vector
def inorder(root, nodes):
    if root is None:
        return
    inorder(root.left, nodes)
    nodes.append(root.data)
    inorder(root.right, nodes)

# Inorder traversal to convert tree to BST.
def constructBst(root, nodes, index):
    if root is None:
        return
    constructBst(root.left, nodes, index)
    
    # Update root value
    root.data = nodes[index[0]]
    index[0] += 1
    
    constructBst(root.right, nodes, index)

# Function to convert a binary tree to a binary search tree
def binaryTreeToBst(root):
    nodes = []
    inorder(root, nodes)

    # sort the nodes
    nodes.sort()

    index = [0]
    constructBst(root, nodes, index)
    return root

# Function to print the inorder traversal of a binary tree
def printInorder(root):
    if root is None:
        return
    printInorder(root.left)
    print(root.data, end=" ")
    printInorder(root.right)

if __name__ == "__main__":
    
    # Creating the tree
    #         10
    #        /  \
    #       2    7
    #      / \
    #     8   4
    root = Node(10)
    root.left = Node(2)
    root.right = Node(7)
    root.left.left = Node(8)
    root.left.right = Node(4)

    ans = binaryTreeToBst(root)
    printInorder(ans)

// C# Program to convert binary 
// tree to binary search tree.
using System;
using System.Collections.Generic;

class Node {
    public int data;
    public Node left, right;

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

class GfG {
    
    // Inorder traversal to store the nodes in a list
    static void Inorder(Node root, List<int> nodes) {
        if (root == null) {
            return;
        }
        Inorder(root.left, nodes);
        nodes.Add(root.data);
        Inorder(root.right, nodes);
    }

    // Inorder traversal to convert tree to BST.
    static void ConstructBST(Node root, 
                             List<int> nodes, ref int index) {
        if (root == null) return;

        ConstructBST(root.left, nodes, ref index);

        // Update root value
        root.data = nodes[index];
        index++;

        ConstructBST(root.right, nodes, ref index);
    }

    // Function to convert a binary tree to a binary search tree
    static Node BinaryTreeToBST(Node root) {
        List<int> nodes = new List<int>();
        Inorder(root, nodes);

        // sort the nodes
        nodes.Sort();

        int index = 0;
        ConstructBST(root, nodes, ref index);
        return root;
    }

    // Function to print the inorder traversal of a binary tree
    static void PrintInorder(Node root) {
        if (root == null) {
            return;
        }
        PrintInorder(root.left);
        Console.Write(root.data + " ");
        PrintInorder(root.right);
    }

    static void Main() {
      
        // Creating the tree
        //         10
        //        /  \
        //       2    7
        //      / \
        //     8   4
        Node root = new Node(10);
        root.left = new Node(2);
        root.right = new Node(7);
        root.left.left = new Node(8);
        root.left.right = new Node(4);

        Node ans = BinaryTreeToBST(root);
        PrintInorder(ans);
    }
}

JavaScript

// JavaScript Program to convert binary 
// tree to binary search tree.
class Node {
    constructor(x) {
        this.data = x;
        this.left = null;
        this.right = null;
    }
}

// Inorder traversal to store the nodes in a vector
function inorderTrav(root, nodes) {
    if (root === null) {
        return;
    }
    inorderTrav(root.left, nodes);
    nodes.push(root.data);
    inorderTrav(root.right, nodes);
}

// Inorder traversal to convert tree to BST.
function constructBST(root, nodes, index) {
    if (root === null) return;

    constructBST(root.left, nodes, index);

    // Update root value
    root.data = nodes[index[0]];
    index[0]++;

    constructBST(root.right, nodes, index);
}

// Function to convert a binary tree to a binary search tree
function binaryTreeToBST(root) {
    let nodes = [];
    inorderTrav(root, nodes);

    // sort the nodes
    nodes.sort((a, b) => a - b);

    let index = [0];
    constructBST(root, nodes, index);
    return root;
}

// Function to print the inorder traversal of a binary tree
function printInorder(root) {
    if (root === null) {
        return;
    }
    printInorder(root.left);
    console.log(root.data);
    printInorder(root.right);
}

// Creating the tree
//         10
//        /  \
//       2    7
//      / \
//     8   4
let root = new Node(10);
root.left = new Node(2);
root.right = new Node(7);
root.left.left = new Node(8);
root.left.right = new Node(4);

let ans = binaryTreeToBST(root);
printInorder(ans);

Output

2 4 7 8 10

Time Complexity: O(nlogn), for sorting the array.
Auxiliary Space: O(n), for storing nodes in an array.

Related article:

Binary Tree to Binary Search Tree Conversion using STL set

Binary Tree to Binary Search Tree Conversion

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