Merge Two Balanced Binary Search Trees

• Difficulty Level : Medium
• Last Updated : 14 Jan, 2022

You are given two balanced binary search trees e.g., AVL or Red-Black Tree. Write a function that merges the two given balanced BSTs into a balanced binary search tree. Let there be m elements in the first tree and n elements in the other tree. Your merge function should take O(m+n) time.
In the following solutions, it is assumed that the sizes of trees are also given as input. If the size is not given, then we can get the size by traversing the tree (See this).

Method 1 (Insert elements of the first tree to second)
Take all elements of first BST one by one, and insert them into the second BST. Inserting an element to a self balancing BST takes Logn time (See this) where n is size of the BST. So time complexity of this method is Log(n) + Log(n+1) … Log(m+n-1). The value of this expression will be between mLogn and mLog(m+n-1). As an optimization, we can pick the smaller tree as first tree.
Method 2 (Merge Inorder Traversals)
1) Do inorder traversal of first tree and store the traversal in one temp array arr1[]. This step takes O(m) time.
2) Do inorder traversal of second tree and store the traversal in another temp array arr2[]. This step takes O(n) time.
3) The arrays created in step 1 and 2 are sorted arrays. Merge the two sorted arrays into one array of size m + n. This step takes O(m+n) time.
4) Construct a balanced tree from the merged array using the technique discussed in this post. This step takes O(m+n) time.
Time complexity of this method is O(m+n) which is better than method 1. This method takes O(m+n) time even if the input BSTs are not balanced.
Following is implementation of this method.

C++

 // C++ program to Merge Two Balanced Binary Search Trees#includeusing namespace std; /* A binary tree node has data, pointer to left childand a pointer to right child */class node{    public:    int data;    node* left;    node* right;}; // A utility function to merge two sorted arrays into oneint *merge(int arr1[], int arr2[], int m, int n); // A helper function that stores inorder// traversal of a tree in inorder arrayvoid storeInorder(node* node, int inorder[],                            int *index_ptr); /* A function that constructs BalancedBinary Search Tree from a sorted arraySee https://www.geeksforgeeks.org/sorted-array-to-balanced-bst/ */node* sortedArrayToBST(int arr[], int start, int end); /* This function merges two balancedBSTs with roots as root1 and root2.m and n are the sizes of the trees respectively */node* mergeTrees(node *root1, node *root2, int m, int n){    // Store inorder traversal of    // first tree in an array arr1[]    int *arr1 = new int[m];    int i = 0;    storeInorder(root1, arr1, &i);     // Store inorder traversal of second    // tree in another array arr2[]    int *arr2 = new int[n];    int j = 0;    storeInorder(root2, arr2, &j);     // Merge the two sorted array into one    int *mergedArr = merge(arr1, arr2, m, n);     // Construct a tree from the merged    // array and return root of the tree    return sortedArrayToBST (mergedArr, 0, m + n - 1);} /* Helper function that allocatesa new node with the given data andNULL left and right pointers. */node* newNode(int data){    node* Node = new node();    Node->data = data;    Node->left = NULL;    Node->right = NULL;     return(Node);} // A utility function to print inorder// traversal of a given binary treevoid printInorder(node* node){    if (node == NULL)        return;     /* first recur on left child */    printInorder(node->left);     cout << node->data << " ";     /* now recur on right child */    printInorder(node->right);} // A utility function to merge// two sorted arrays into oneint *merge(int arr1[], int arr2[], int m, int n){    // mergedArr[] is going to contain result    int *mergedArr = new int[m + n];    int i = 0, j = 0, k = 0;     // Traverse through both arrays    while (i < m && j < n)    {        // Pick the smaller element and put it in mergedArr        if (arr1[i] < arr2[j])        {            mergedArr[k] = arr1[i];            i++;        }        else        {            mergedArr[k] = arr2[j];            j++;        }        k++;    }     // If there are more elements in first array    while (i < m)    {        mergedArr[k] = arr1[i];        i++; k++;    }     // If there are more elements in second array    while (j < n)    {        mergedArr[k] = arr2[j];        j++; k++;    }     return mergedArr;} // A helper function that stores inorder// traversal of a tree rooted with nodevoid storeInorder(node* node, int inorder[], int *index_ptr){    if (node == NULL)        return;     /* first recur on left child */    storeInorder(node->left, inorder, index_ptr);     inorder[*index_ptr] = node->data;    (*index_ptr)++; // increase index for next entry     /* now recur on right child */    storeInorder(node->right, inorder, index_ptr);} /* A function that constructs Balanced// Binary Search Tree from a sorted arraySee https://www.geeksforgeeks.org/sorted-array-to-balanced-bst/ */node* sortedArrayToBST(int arr[], int start, int end){    /* Base Case */    if (start > end)    return NULL;     /* Get the middle element and make it root */    int mid = (start + end)/2;    node *root = newNode(arr[mid]);     /* Recursively construct the left subtree and make it    left child of root */    root->left = sortedArrayToBST(arr, start, mid-1);     /* Recursively construct the right subtree and make it    right child of root */    root->right = sortedArrayToBST(arr, mid+1, end);     return root;} /* Driver code*/int main(){    /* Create following tree as first balanced BST        100        / \        50 300    / \    20 70    */    node *root1 = newNode(100);    root1->left     = newNode(50);    root1->right = newNode(300);    root1->left->left = newNode(20);    root1->left->right = newNode(70);     /* Create following tree as second balanced BST            80        / \        40 120    */    node *root2 = newNode(80);    root2->left     = newNode(40);    root2->right = newNode(120);     node *mergedTree = mergeTrees(root1, root2, 5, 3);     cout << "Following is Inorder traversal of the merged tree \n";    printInorder(mergedTree);     return 0;} // This code is contributed by rathbhupendra

C

 // C program to Merge Two Balanced Binary Search Trees#include #include  /* A binary tree node has data, pointer to left child   and a pointer to right child */struct node{    int data;    struct node* left;    struct node* right;}; // A utility function to merge two sorted arrays into oneint *merge(int arr1[], int arr2[], int m, int n); // A helper function that stores inorder traversal of a tree in inorder arrayvoid storeInorder(struct node* node, int inorder[], int *index_ptr); /* A function that constructs Balanced Binary Search Tree from a sorted arraystruct node* sortedArrayToBST(int arr[], int start, int end); /* This function merges two balanced BSTs with roots as root1 and root2.   m and n are the sizes of the trees respectively */struct node* mergeTrees(struct node *root1, struct node *root2, int m, int n){    // Store inorder traversal of first tree in an array arr1[]    int *arr1 = new int[m];    int i = 0;    storeInorder(root1, arr1, &i);     // Store inorder traversal of second tree in another array arr2[]    int *arr2 = new int[n];    int j = 0;    storeInorder(root2, arr2, &j);     // Merge the two sorted array into one    int *mergedArr = merge(arr1, arr2, m, n);     // Construct a tree from the merged array and return root of the tree    return sortedArrayToBST (mergedArr, 0, m+n-1);} /* Helper function that allocates a new node with the   given data and NULL left and right pointers. */struct node* newNode(int data){    struct node* node = (struct node*)                        malloc(sizeof(struct node));    node->data = data;    node->left = NULL;    node->right = NULL;     return(node);} // A utility function to print inorder traversal of a given binary treevoid printInorder(struct node* node){    if (node == NULL)        return;     /* first recur on left child */    printInorder(node->left);     printf("%d ", node->data);     /* now recur on right child */    printInorder(node->right);} // A utility function to merge two sorted arrays into oneint *merge(int arr1[], int arr2[], int m, int n){    // mergedArr[] is going to contain result    int *mergedArr = new int[m + n];    int i = 0, j = 0, k = 0;     // Traverse through both arrays    while (i < m && j < n)    {        // Pick the smaller element and put it in mergedArr        if (arr1[i] < arr2[j])        {            mergedArr[k] = arr1[i];            i++;        }        else        {            mergedArr[k] = arr2[j];            j++;        }        k++;    }     // If there are more elements in first array    while (i < m)    {        mergedArr[k] = arr1[i];        i++; k++;    }     // If there are more elements in second array    while (j < n)    {        mergedArr[k] = arr2[j];        j++; k++;    }     return mergedArr;} // A helper function that stores inorder traversal of a tree rooted with nodevoid storeInorder(struct node* node, int inorder[], int *index_ptr){    if (node == NULL)        return;     /* first recur on left child */    storeInorder(node->left, inorder, index_ptr);     inorder[*index_ptr] = node->data;    (*index_ptr)++;  // increase index for next entry     /* now recur on right child */    storeInorder(node->right, inorder, index_ptr);} /* A function that constructs Balanced Binary Search Tree from a sorted arraystruct node* sortedArrayToBST(int arr[], int start, int end){    /* Base Case */    if (start > end)      return NULL;     /* Get the middle element and make it root */    int mid = (start + end)/2;    struct node *root = newNode(arr[mid]);     /* Recursively construct the left subtree and make it       left child of root */    root->left =  sortedArrayToBST(arr, start, mid-1);     /* Recursively construct the right subtree and make it       right child of root */    root->right = sortedArrayToBST(arr, mid+1, end);     return root;} /* Driver program to test above functions*/int main(){    /* Create following tree as first balanced BST           100          /  \        50    300       / \      20  70    */    struct node *root1  = newNode(100);    root1->left         = newNode(50);    root1->right        = newNode(300);    root1->left->left   = newNode(20);    root1->left->right  = newNode(70);     /* Create following tree as second balanced BST            80           /  \         40   120    */    struct node *root2  = newNode(80);    root2->left         = newNode(40);    root2->right        = newNode(120);     struct node *mergedTree = mergeTrees(root1, root2, 5, 3);     printf ("Following is Inorder traversal of the merged tree \n");    printInorder(mergedTree);     getchar();    return 0;}

Java

 // Java program to Merge Two Balanced Binary Search Treesimport java.io.*;import java.util.ArrayList; // A binary tree nodeclass Node {         int data;    Node left, right;         Node(int d) {        data = d;        left = right = null;    }} class BinarySearchTree{         // Root of BST    Node root;     // Constructor    BinarySearchTree() {        root = null;    }         // Inorder traversal of the tree    void inorder()    {        inorderUtil(this.root);    }     // Utility function for inorder traversal of the treevoid inorderUtil(Node node){    if(node==null)        return;             inorderUtil(node.left);    System.out.print(node.data + " ");    inorderUtil(node.right);}          // A Utility Method that stores inorder traversal of a tree    public ArrayList storeInorderUtil(Node node, ArrayList list)    {        if(node == null)            return list;                 //recur on the left child        storeInorderUtil(node.left, list);                 // Adds data to the list        list.add(node.data);                 //recur on the right child        storeInorderUtil(node.right, list);                 return list;    }         // Method that stores inorder traversal of a tree    ArrayList storeInorder(Node node)    {        ArrayList list1 = new ArrayList<>();        ArrayList list2 = storeInorderUtil(node,list1);        return list2;    }     // Method that merges two ArrayLists into one.    ArrayList merge(ArrayListlist1, ArrayListlist2, int m, int n)    {        // list3 will contain the merge of list1 and list2        ArrayList list3 = new ArrayList<>();        int i=0;        int j=0;                 //Traversing through both ArrayLists        while( ilist, int start, int end)    {        // Base case        if(start > end)            return null;             // Get the middle element and make it root            int mid = (start+end)/2;        Node node = new Node(list.get(mid));         /* Recursively construct the left subtree and make it        left child of root */        node.left = ALtoBST(list, start, mid-1);                 /* Recursively construct the right subtree and make it        right child of root */        node.right = ALtoBST(list, mid+1, end);         return node;    }         // Method that merges two trees into a single one.    Node mergeTrees(Node node1, Node node2)    {        //Stores Inorder of tree1 to list1        ArrayListlist1 = storeInorder(node1);                 //Stores Inorder of tree2 to list2        ArrayListlist2 = storeInorder(node2);                 // Merges both list1 and list2 into list3        ArrayListlist3 = merge(list1, list2, list1.size(), list2.size());                 //Eventually converts the merged list into resultant BST        Node node = ALtoBST(list3, 0, list3.size()-1);        return node;    }     // Driver function    public static void main (String[] args)    {                 /* Creating following tree as First balanced BST                100                / \                50 300                / \               20 70        */                 BinarySearchTree tree1 = new BinarySearchTree();        tree1.root = new Node(100);        tree1.root.left = new Node(50);        tree1.root.right = new Node(300);        tree1.root.left.left = new Node(20);        tree1.root.left.right = new Node(70);                 /* Creating following tree as second balanced BST                80                / \              40 120        */                     BinarySearchTree tree2 = new BinarySearchTree();        tree2.root = new Node(80);           tree2.root.left = new Node(40);        tree2.root.right = new Node(120);                                  BinarySearchTree tree = new BinarySearchTree();           tree.root = tree.mergeTrees(tree1.root, tree2.root);        System.out.println("The Inorder traversal of the merged BST is: ");        tree.inorder();    }}// This code has been contributed by Kamal Rawal

Python3

 # A binary tree node has data, pointer to left child # and a pointer to right childclass Node:    def __init__(self, val):        self.val = val        self.left = None        self.right = None # A utility function to merge two sorted arrays into one# Time Complexity of below function: O(m + n)# Space Complexity of below function: O(m + n)def merge_sorted_arr(arr1, arr2):    arr = []    i = j = 0    while i < len(arr1) and j < len(arr2):        if arr1[i] <= arr2[j]:            arr.append(arr1[i])            i += 1        else:            arr.append(arr2[j])            j += 1    while i < len(arr1):        arr.append(arr1[i])        i += 1    while i < len(arr2):        arr.append(arr2[j])        j += 1    return arr # A helper function that stores inorder# traversal of a tree in arrdef inorder(root, arr = []):    if root:        inorder(root.left, arr)        arr.append(root.val)        inorder(root.right, arr) # A utility function to insert the values# in the individual Treedef insert(root, val):    if not root:        return Node(val)    if root.val == val:        return root    elif root.val > val:        root.left = insert(root.left, val)    else:        root.right = insert(root.right, val)    return root # Converts the merged array to a balanced BST# Explanation of the below code:# https://www.geeksforgeeks.org/sorted-array-to-balanced-bst/def arr_to_bst(arr):    if not arr:        return None    mid = len(arr) // 2    root = Node(arr[mid])    root.left = arr_to_bst(arr[:mid])    root.right = arr_to_bst(arr[mid + 1:])    return root if __name__=='__main__':    root1 = root2 = None         # Inserting values in first tree    root1 = insert(root1, 100)    root1 = insert(root1, 50)    root1 = insert(root1, 300)    root1 = insert(root1, 20)    root1 = insert(root1, 70)         # Inserting values in second tree    root2 = insert(root2, 80)    root2 = insert(root2, 40)    root2 = insert(root2, 120)    arr1 = []    inorder(root1, arr1)    arr2 = []    inorder(root2, arr2)    arr = merge_sorted_arr(arr1, arr2)    root = arr_to_bst(arr)    res = []    inorder(root, res)    print('Following is Inorder traversal of the merged tree')    for i in res:      print(i, end = ' ')       # This code is contributed by Flarow4

C#

 // C# program to Merge Two Balanced Binary Search Treesusing System;using System.Collections.Generic; // A binary tree nodepublic class Node{     public int data;    public Node left, right;     public Node(int d)    {        data = d;        left = right = null;    }} public class BinarySearchTree{     // Root of BST    public Node root;     // Constructor    public BinarySearchTree()    {        root = null;    }     // Inorder traversal of the tree    public virtual void inorder()    {        inorderUtil(this.root);    } // Utility function for inorder traversal of the treepublic virtual void inorderUtil(Node node){    if (node == null)    {        return;    }     inorderUtil(node.left);    Console.Write(node.data + " ");    inorderUtil(node.right);}      // A Utility Method that stores inorder traversal of a tree    public virtual List storeInorderUtil(Node node, List list)    {        if (node == null)        {            return list;        }         //recur on the left child        storeInorderUtil(node.left, list);         // Adds data to the list        list.Add(node.data);         //recur on the right child        storeInorderUtil(node.right, list);         return list;    }     // Method that stores inorder traversal of a tree    public virtual List storeInorder(Node node)    {        List list1 = new List();        List list2 = storeInorderUtil(node,list1);        return list2;    }     // Method that merges two ArrayLists into one.     public virtual List merge(List list1, List list2, int m, int n)    {        // list3 will contain the merge of list1 and list2        List list3 = new List();        int i = 0;        int j = 0;         //Traversing through both ArrayLists        while (i < m && j < n)        {            // Smaller one goes into list3            if (list1[i] < list2[j])            {                list3.Add(list1[i]);                i++;            }            else            {                list3.Add(list2[j]);                j++;            }        }         // Adds the remaining elements of list1 into list3        while (i < m)        {            list3.Add(list1[i]);            i++;        }         // Adds the remaining elements of list2 into list3        while (j < n)        {            list3.Add(list2[j]);            j++;        }        return list3;    }     // Method that converts an ArrayList to a BST    public virtual Node ALtoBST(List list, int start, int end)    {        // Base case        if (start > end)        {            return null;        }         // Get the middle element and make it root             int mid = (start + end) / 2;        Node node = new Node(list[mid]);         /* Recursively construct the left subtree and make it        left child of root */        node.left = ALtoBST(list, start, mid - 1);         /* Recursively construct the right subtree and make it        right child of root */        node.right = ALtoBST(list, mid + 1, end);     return node;    }     // Method that merges two trees into a single one.     public virtual Node mergeTrees(Node node1, Node node2)    {        //Stores Inorder of tree1 to list1        List list1 = storeInorder(node1);         //Stores Inorder of tree2 to list2        List list2 = storeInorder(node2);         // Merges both list1 and list2 into list3        List list3 = merge(list1, list2, list1.Count, list2.Count);         //Eventually converts the merged list into resultant BST        Node node = ALtoBST(list3, 0, list3.Count - 1);        return node;    }     // Driver function    public static void Main(string[] args)    {         /* Creating following tree as First balanced BST                100                / \                50 300                / \               20 70        */         BinarySearchTree tree1 = new BinarySearchTree();        tree1.root = new Node(100);        tree1.root.left = new Node(50);        tree1.root.right = new Node(300);        tree1.root.left.left = new Node(20);        tree1.root.left.right = new Node(70);         /* Creating following tree as second balanced BST                80                / \              40 120        */         BinarySearchTree tree2 = new BinarySearchTree();        tree2.root = new Node(80);        tree2.root.left = new Node(40);        tree2.root.right = new Node(120);          BinarySearchTree tree = new BinarySearchTree();        tree.root = tree.mergeTrees(tree1.root, tree2.root);        Console.WriteLine("The Inorder traversal of the merged BST is: ");        tree.inorder();    }}   // This code is contributed by Shrikant13

Javascript



Output:

Following is Inorder traversal of the merged tree
20 40 50 70 80 100 120 300

Method 3 (In-Place Merge using DLL)
We can use a Doubly Linked List to merge trees in place. Following are the steps.
1) Convert the given two Binary Search Trees into doubly linked list in place (Refer this post for this step).
2) Merge the two sorted Linked Lists (Refer this post for this step).
3) Build a Balanced Binary Search Tree from the merged list created in step 2. (Refer this post for this step)
Time complexity of this method is also O(m+n) and this method does conversion in place.
Thanks to Dheeraj and Ronzii for suggesting this method.