Print Common Nodes in Two Binary Search Trees

Given two Binary Search Trees, find common nodes in them. In other words, find the intersection of two BSTs. 

Example: 

Input: root1:
                   5
             /          \
         1              10
     /      \          /
  0        4       7
                      \
                       9

root2:    10
           /       \
        7         20
    /      \
4          9
Output: 4 7 9 10      

Naive Approach:

A simple way is to one by one search every node of the first tree in the second tree. 

Time Complexity: O(M * H) where M is the number of nodes in the first tree and H is the height of the second tree.
Auxiliary Space: O(1)

Common Nodes in Two Binary Search Trees using Inorder and Intersection:

The idea is simply to take the inorder traversal of both the trees and store them in two separate arrays and then find the intersection between two arrays.

Follow the steps below to solve the problem:

• Do inorder traversal of the first tree and store the traversal in an auxiliary array ar1[]. See sortedInorder() here. 
• Do inorder traversal of the second tree and store the traversal in an auxiliary array ar2[] 
• Find intersection of ar1[] and ar2[].

Below is the implementation of the above approach:

C++

// C++ program of iterative traversal based method to
// find common elements in two BSTs.

#include <iostream>
#include <stack>
#include <vector>
using namespace std;

// A BST node
struct Node {
    int key;
    struct Node *left, *right;
};

// A utility function to create a new node
Node* newNode(int ele)
{
    Node* temp = new Node;
    temp->key = ele;
    temp->left = temp->right = NULL;
    return temp;
}

// A utility function to do inorder traversal
void inorder(struct Node* root, vector<int> &traversal)
{
    if (root) {
        inorder(root->left, traversal);
        traversal.push_back(root->key);
        inorder(root->right, traversal);
    }
}

// Function two print common elements in given two trees
void printCommon(Node* root1, Node* root2)
{
    vector<int> inorder1, inorder2;
    // Storing inorder traversal of both the trees
    inorder(root1, inorder1);
    inorder(root2, inorder2);

    cout << "Tree 1 : " << endl;
    for(int i = 0; i < inorder1.size(); i++){
        cout << inorder1[i] << " ";
    }
    cout << endl;

    cout << "Tree 2 : " << endl;
    for(int i = 0; i < inorder2.size(); i++){
        cout << inorder2[i] << " ";
    }
    cout << endl;

    cout << "Common Nodes: " << endl;

    // Using two pointers calculating common nodes in both the traversals
    int i = 0, j = 0;
    while(i < inorder1.size() && j < inorder2.size()){
        if(inorder1[i] == inorder2[j]){
            cout << inorder1[i] << " ";
            i++;
            j++;
        }
        else if(inorder1[i] < inorder2[j]){
            i++;
        }
        else{
            j++;
        }
    }
}

// A utility function to insert a new Node
// with given key in BST
struct Node* insert(struct Node* node, int key)
{
    // If the tree is empty, return a new Node
    if (node == NULL)
        return newNode(key);

    // Otherwise, recur down the tree
    if (key < node->key)
        node->left = insert(node->left, key);
    else if (key > node->key)
        node->right = insert(node->right, key);

    // Return the (unchanged) Node pointer
    return node;
}

// Driver program
int main()
{
    // Create first tree as shown in example
    Node* root1 = NULL;
    root1 = insert(root1, 5);
    root1 = insert(root1, 1);
    root1 = insert(root1, 10);
    root1 = insert(root1, 0);
    root1 = insert(root1, 4);
    root1 = insert(root1, 7);
    root1 = insert(root1, 9);

    // Create second tree as shown in example
    Node* root2 = NULL;
    root2 = insert(root2, 10);
    root2 = insert(root2, 7);
    root2 = insert(root2, 20);
    root2 = insert(root2, 4);
    root2 = insert(root2, 9);

    printCommon(root1, root2);

    return 0;
}

Java

//Java code for the above approach
import java.util.*;

class Node {
    int key;
    Node left, right;

    public Node(int item)
    {
        key = item;
        left = right = null;
    }
}

class BinaryTree {

    Node root1, root2;

    // A utility function to do inorder traversal
    void inorder(Node root, ArrayList<Integer> traversal)
    {
        if (root != null) {
            inorder(root.left, traversal);
            traversal.add(root.key);
            inorder(root.right, traversal);
        }
    }

    // Function two print common elements in given two trees
    void printCommon()
    {
        ArrayList<Integer> inorder1
            = new ArrayList<Integer>();
        ArrayList<Integer> inorder2
            = new ArrayList<Integer>();
        // Storing inorder traversal of both the trees
        inorder(root1, inorder1);
        inorder(root2, inorder2);

        System.out.println("Tree 1 : ");
        for (int i = 0; i < inorder1.size(); i++) {
            System.out.print(inorder1.get(i) + " ");
        }
        System.out.println();

        System.out.println("Tree 2 : ");
        for (int i = 0; i < inorder2.size(); i++) {
            System.out.print(inorder2.get(i) + " ");
        }
        System.out.println();

        System.out.println("Common Nodes: ");

        // Using two pointers calculating common nodes in
        // both the traversals
        int i = 0, j = 0;
        while (i < inorder1.size() && j < inorder2.size()) {
            if (inorder1.get(i) == inorder2.get(j)) {
                System.out.print(inorder1.get(i) + " ");
                i++;
                j++;
            }
            else if (inorder1.get(i) < inorder2.get(j)) {
                i++;
            }
            else {
                j++;
            }
        }
    }

    // A utility function to insert a new Node
    // with given key in BST
    Node insert(Node node, int key)
    {
        // If the tree is empty, return a new Node
        if (node == null) {
            return new Node(key);
        }

        // Otherwise, recur down the tree
        if (key < node.key) {
            node.left = insert(node.left, key);
        }
        else if (key > node.key) {
            node.right = insert(node.right, key);
        }

        // Return the (unchanged) Node pointer
        return node;
    }

    public static void main(String[] args)
    {
        BinaryTree tree = new BinaryTree();

        // Create first tree as shown in example
        tree.root1 = tree.insert(tree.root1, 5);
        tree.root1 = tree.insert(tree.root1, 1);
        tree.root1 = tree.insert(tree.root1, 10);
        tree.root1 = tree.insert(tree.root1, 0);
        tree.root1 = tree.insert(tree.root1, 4);
        tree.root1 = tree.insert(tree.root1, 7);
        tree.root1 = tree.insert(tree.root1, 9);

        // Create second tree as shown in example
        tree.root2 = tree.insert(tree.root2, 10);
        tree.root2 = tree.insert(tree.root2, 7);
        tree.root2 = tree.insert(tree.root2, 20);
        tree.root2 = tree.insert(tree.root2, 4);
        tree.root2 = tree.insert(tree.root2, 9);

        tree.printCommon();
    }
}

Output

Tree 1 : 
0 1 4 5 7 9 10 
Tree 2 : 
4 7 9 10 20 
Common Nodes: 
4 7 9 10 

Time Complexity: O(M + N), Here ‘M’ and ‘N’ are the number of nodes in the first and second trees respectively.
Auxiliary Space: O(M + N), Need two separate arrays for storing inorder traversals of both the trees.

(Linear Time and limited Extra Space) using iterative inorder traversal:

The idea is to use two stacks and store inorder traversal of trees in respective stacks but the maximum number of elements should be equal to that particular branch of the tree.

Follow the steps below to solve the problem:

• Create two stacks for two inorder traversals
• Push the Nodes of the first tree in stack s1, till that branch reaches NULL.
• Push the Nodes of the second tree in stack s2, till that branch reaches NULL
• If both branches reach NULL, then check 
  • If root1->data < root2->data, 
    • If node of the first tree is smaller than that of the second tree, then it is obvious that the inorder successors of the current node can have the same value as that of the second tree Node. 
    • Thus, we pop from s1.

Below is the implementation of the above approach.

C++

// C++ program of iterative traversal based method to
// find common elements in two BSTs.

#include <iostream>
#include <stack>
using namespace std;

// A BST node
struct Node {
    int key;
    struct Node *left, *right;
};

// A utility function to create a new node
Node* newNode(int ele)
{
    Node* temp = new Node;
    temp->key = ele;
    temp->left = temp->right = NULL;
    return temp;
}

// Function two print common elements in given two trees
void printCommon(Node* root1, Node* root2)
{
    // Create two stacks for two inorder traversals
    stack<Node*> stack1, s1, s2;

    while (1) {
        // Push the Nodes of first tree in stack s1
        if (root1) {
            s1.push(root1);
            root1 = root1->left;
        }

        // Push the Nodes of second tree in stack s2
        else if (root2) {
            s2.push(root2);
            root2 = root2->left;
        }

        // Both root1 and root2 are NULL here
        else if (!s1.empty() && !s2.empty()) {
            root1 = s1.top();
            root2 = s2.top();

            // If current keys in two trees are same
            if (root1->key == root2->key) {
                cout << root1->key << " ";
                s1.pop();
                s2.pop();

                // Move to the inorder successor
                root1 = root1->right;
                root2 = root2->right;
            }

            else if (root1->key < root2->key) {
                // If Node of first tree is smaller, than
                // that of second tree, then its obvious
                // that the inorder successors of current
                // node can have same value as that of the
                // second tree Node. Thus, we pop from s2
                s1.pop();
                root1 = root1->right;

                // root2 is set to NULL, because we need
                // new Nodes of tree 1
                root2 = NULL;
            }
            else if (root1->key > root2->key) {
                s2.pop();
                root2 = root2->right;
                root1 = NULL;
            }
        }

        // Both roots and both stacks are empty
        else
            break;
    }
}

// A utility function to do inorder traversal
void inorder(struct Node* root)
{
    if (root) {
        inorder(root->left);
        cout << root->key << " ";
        inorder(root->right);
    }
}

// A utility function to insert a new Node
// with given key in BST
struct Node* insert(struct Node* node, int key)
{
    // If the tree is empty, return a new Node
    if (node == NULL)
        return newNode(key);

    // Otherwise, recur down the tree
    if (key < node->key)
        node->left = insert(node->left, key);
    else if (key > node->key)
        node->right = insert(node->right, key);

    // Return the (unchanged) Node pointer
    return node;
}

// Driver program
int main()
{
    // Create first tree as shown in example
    Node* root1 = NULL;
    root1 = insert(root1, 5);
    root1 = insert(root1, 1);
    root1 = insert(root1, 10);
    root1 = insert(root1, 0);
    root1 = insert(root1, 4);
    root1 = insert(root1, 7);
    root1 = insert(root1, 9);

    // Create second tree as shown in example
    Node* root2 = NULL;
    root2 = insert(root2, 10);
    root2 = insert(root2, 7);
    root2 = insert(root2, 20);
    root2 = insert(root2, 4);
    root2 = insert(root2, 9);

    cout << "Tree 1 : " << endl;
    inorder(root1);
    cout << endl;

    cout << "Tree 2 : " << endl;
    inorder(root2);

    cout << "\nCommon Nodes: " << endl;
    printCommon(root1, root2);

    return 0;
}

Java

// Java program of iterative traversal based method to
// find common elements in two BSTs.
import java.util.*;
class GfG {

    // A BST node
    static class Node {
        int key;
        Node left, right;
    }

    // A utility function to create a new node
    static Node newNode(int ele)
    {
        Node temp = new Node();
        temp.key = ele;
        temp.left = null;
        temp.right = null;
        return temp;
    }

    // Function two print common elements in given two trees
    static void printCommon(Node root1, Node root2)
    {

        Stack<Node> s1 = new Stack<Node>();
        Stack<Node> s2 = new Stack<Node>();

        while (true) {
            // Push the Nodes of first tree in stack s1
            if (root1 != null) {
                s1.push(root1);
                root1 = root1.left;
            }

            // Push the Nodes of second tree in stack s2
            else if (root2 != null) {
                s2.push(root2);
                root2 = root2.left;
            }

            // Both root1 and root2 are NULL here
            else if (!s1.isEmpty() && !s2.isEmpty()) {
                root1 = s1.peek();
                root2 = s2.peek();

                // If current keys in two trees are same
                if (root1.key == root2.key) {
                    System.out.print(root1.key + " ");
                    s1.pop();
                    s2.pop();

                    // Move to the inorder successor
                    root1 = root1.right;
                    root2 = root2.right;
                }

                else if (root1.key < root2.key) {
                    // If Node of first tree is smaller,
                    // than that of second tree, then its
                    // obvious that the inorder successors
                    // of current Node can have same value
                    // as that of the second tree Node.
                    // Thus, we pop from s2
                    s1.pop();
                    root1 = root1.right;

                    // root2 is set to NULL, because we need
                    // new Nodes of tree 1
                    root2 = null;
                }
                else if (root1.key > root2.key) {
                    s2.pop();
                    root2 = root2.right;
                    root1 = null;
                }
            }

            // Both roots and both stacks are empty
            else
                break;
        }
    }

    // A utility function to do inorder traversal
    static void inorder(Node root)
    {
        if (root != null) {
            inorder(root.left);
            System.out.print(root.key + " ");
            inorder(root.right);
        }
    }

    // A utility function to insert a new Node
    // with given key in BST
    static Node insert(Node node, int key)
    {
        // If the tree is empty, return a new Node
        if (node == null)
            return newNode(key);

        // Otherwise, recur down the tree
        if (key < node.key)
            node.left = insert(node.left, key);
        else if (key > node.key)
            node.right = insert(node.right, key);

        // Return the (unchanged) Node pointer
        return node;
    }

    // Driver program
    public static void main(String[] args)
    {
        // Create first tree as shown in example
        Node root1 = null;
        root1 = insert(root1, 5);
        root1 = insert(root1, 1);
        root1 = insert(root1, 10);
        root1 = insert(root1, 0);
        root1 = insert(root1, 4);
        root1 = insert(root1, 7);
        root1 = insert(root1, 9);

        // Create second tree as shown in example
        Node root2 = null;
        root2 = insert(root2, 10);
        root2 = insert(root2, 7);
        root2 = insert(root2, 20);
        root2 = insert(root2, 4);
        root2 = insert(root2, 9);

        System.out.print("Tree 1 : "
                         + "\n");
        inorder(root1);
        System.out.println();
        System.out.print("Tree 2 : "
                         + "\n");
        inorder(root2);
        System.out.println();
        System.out.println("Common Nodes: ");

        printCommon(root1, root2);
    }
}

Python3

# Python3 program of iterative traversal based
# method to find common elements in two BSTs.

# A utility function to create a new node


class newNode:
    def __init__(self, key):
        self.key = key
        self.left = self.right = None

# Function two print common elements
# in given two trees


def printCommon(root1, root2):

    # Create two stacks for two inorder
    # traversals
    s1 = []
    s2 = []

    while 1:

        # append the Nodes of first
        # tree in stack s1
        if root1:
            s1.append(root1)
            root1 = root1.left

        # append the Nodes of second tree
        # in stack s2
        elif root2:
            s2.append(root2)
            root2 = root2.left

        # Both root1 and root2 are NULL here
        elif len(s1) != 0 and len(s2) != 0:
            root1 = s1[-1]
            root2 = s2[-1]

            # If current keys in two trees are same
            if root1.key == root2.key:
                print(root1.key, end=" ")
                s1.pop(-1)
                s2.pop(-1)

                # move to the inorder successor
                root1 = root1.right
                root2 = root2.right

            elif root1.key < root2.key:

                # If Node of first tree is smaller, than
                # that of second tree, then its obvious
                # that the inorder successors of current
                # Node can have same value as that of the
                # second tree Node. Thus, we pop from s2
                s1.pop(-1)
                root1 = root1.right

                # root2 is set to NULL, because we need
                # new Nodes of tree 1
                root2 = None
            elif root1.key > root2.key:
                s2.pop(-1)
                root2 = root2.right
                root1 = None

        # Both roots and both stacks are empty
        else:
            break

# A utility function to do inorder traversal


def inorder(root):
    if root:
        inorder(root.left)
        print(root.key, end=" ")
        inorder(root.right)

# A utility function to insert a new Node
# with given key in BST


def insert(node, key):

    # If the tree is empty, return a new Node
    if node == None:
        return newNode(key)

    # Otherwise, recur down the tree
    if key < node.key:
        node.left = insert(node.left, key)
    elif key > node.key:
        node.right = insert(node.right, key)

    # return the (unchanged) Node pointer
    return node


# Driver Code
if __name__ == '__main__':

    # Create first tree as shown in example
    root1 = None
    root1 = insert(root1, 5)
    root1 = insert(root1, 1)
    root1 = insert(root1, 10)
    root1 = insert(root1, 0)
    root1 = insert(root1, 4)
    root1 = insert(root1, 7)
    root1 = insert(root1, 9)

    # Create second tree as shown in example
    root2 = None
    root2 = insert(root2, 10)
    root2 = insert(root2, 7)
    root2 = insert(root2, 20)
    root2 = insert(root2, 4)
    root2 = insert(root2, 9)

    print("Tree 1 : ")
    inorder(root1)
    print()

    print("Tree 2 : ")
    inorder(root2)
    print()

    print("Common Nodes: ")
    printCommon(root1, root2)

# This code is contributed by PranchalK

C#

using System;
using System.Collections.Generic;

// C# program of iterative traversal based method to
// find common elements in two BSTs.
public class GfG {

    // A BST node
    public class Node {
        public int key;
        public Node left, right;
    }

    // A utility function to create a new node
    public static Node newNode(int ele)
    {
        Node temp = new Node();
        temp.key = ele;
        temp.left = null;
        temp.right = null;
        return temp;
    }

    // Function two print common elements in given two trees
    public static void printCommon(Node root1, Node root2)
    {
        Stack<Node> s1 = new Stack<Node>();
        Stack<Node> s2 = new Stack<Node>();

        while (true) {
            // push the Nodes of first tree in stack s1
            if (root1 != null) {
                s1.Push(root1);
                root1 = root1.left;
            }

            // push the Nodes of second tree in stack s2
            else if (root2 != null) {
                s2.Push(root2);
                root2 = root2.left;
            }

            // Both root1 and root2 are NULL here
            else if (s1.Count > 0 && s2.Count > 0) {
                root1 = s1.Peek();
                root2 = s2.Peek();

                // If current keys in two trees are same
                if (root1.key == root2.key) {
                    Console.Write(root1.key + " ");
                    s1.Pop();
                    s2.Pop();

                    // move to the inorder successor
                    root1 = root1.right;
                    root2 = root2.right;
                }

                else if (root1.key < root2.key) {
                    // If Node of first tree is smaller,
                    // than that of second tree, then its
                    // obvious that the inorder successors
                    // of current Node can have same value
                    // as that of the second tree Node.
                    // Thus, we pop from s2
                    s1.Pop();
                    root1 = root1.right;

                    // root2 is set to NULL, because we need
                    // new Nodes of tree 1
                    root2 = null;
                }
                else if (root1.key > root2.key) {
                    s2.Pop();
                    root2 = root2.right;
                    root1 = null;
                }
            }

            // Both roots and both stacks are empty
            else {
                break;
            }
        }
    }

    // A utility function to do inorder traversal
    public static void inorder(Node root)
    {
        if (root != null) {
            inorder(root.left);
            Console.Write(root.key + " ");
            inorder(root.right);
        }
    }

    /* A utility function to insert a new Node with given
     * key in BST */
    public static Node insert(Node node, int key)
    {
        /* If the tree is empty, return a new Node */
        if (node == null) {
            return newNode(key);
        }

        /* Otherwise, recur down the tree */
        if (key < node.key) {
            node.left = insert(node.left, key);
        }
        else if (key > node.key) {
            node.right = insert(node.right, key);
        }

        /* return the (unchanged) Node pointer */
        return node;
    }

    // Driver program
    public static void Main(string[] args)
    {
        // Create first tree as shown in example
        Node root1 = null;
        root1 = insert(root1, 5);
        root1 = insert(root1, 1);
        root1 = insert(root1, 10);
        root1 = insert(root1, 0);
        root1 = insert(root1, 4);
        root1 = insert(root1, 7);
        root1 = insert(root1, 9);

        // Create second tree as shown in example
        Node root2 = null;
        root2 = insert(root2, 10);
        root2 = insert(root2, 7);
        root2 = insert(root2, 20);
        root2 = insert(root2, 4);
        root2 = insert(root2, 9);

        Console.Write("Tree 1 : "
                      + "\n");
        inorder(root1);
        Console.WriteLine();
        Console.Write("Tree 2 : "
                      + "\n");
        inorder(root2);
        Console.WriteLine();
        Console.Write("Common Nodes: "
                      + "\n");

        printCommon(root1, root2);
    }
}

// This code is contributed by Shrikant13

Output

Tree 1 : 
0 1 4 5 7 9 10 
Tree 2 : 
4 7 9 10 20 
Common Nodes: 
4 7 9 10 

Time Complexity: O(N+M). Here ‘M’ and ‘N’ are the number of nodes in the first and second trees respectively
Auxiliary Space: O(h1 + h2), Where h1 and h2 are the heights of the first and second tree respectively.

This article is contributed by Ekta Goel. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.