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Largest number in BST which is less than or equal to N

  • Difficulty Level : Easy
  • Last Updated : 05 Jul, 2021

We have a binary search tree and a number N. Our task is to find the greatest number in the binary search tree that is less than or equal to N. Print the value of the element if it exists otherwise print -1. 

BST

Examples: 
For the above given binary search tree- 

Input : N = 24
Output :result = 21
(searching for 24 will be like-5->12->21)

Input  : N = 4
Output : result = 3
(searching for 4 will be like-5->2->3)

We follow recursive approach for solving this problem. We start searching for element from root node. If we reach a leaf and its value is greater than N, element does not exist so return -1. Else if node’s value is less than or equal to N and right value is NULL or greater than N, then return the node value as it will be the answer. 

Otherwise if node’s value is greater than N, then search for the element in the left subtree else search for the element in the right subtree by calling the same function by passing the left or right values accordingly. 

C++




// C++ code to find the largest value smaller
// than or equal to N
#include <bits/stdc++.h>
using namespace std;
 
struct Node {
    int key;
    Node *left, *right;
};
 
// To create new BST Node
Node* newNode(int item)
{
    Node* temp = new Node;
    temp->key = item;
    temp->left = temp->right = NULL;
    return temp;
}
 
// To insert a new node in BST
Node* insert(Node* node, int key)
{
    // if tree is empty return new node
    if (node == NULL)
        return newNode(key);
 
    // if key is less then or greater then
    // node value then 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;
}
 
// function to find max value less then N
int findMaxforN(Node* root, int N)
{
    // Base cases
    if (root == NULL)
        return -1;
    if (root->key == N)
        return N;
 
    // If root's value is smaller, try in right
    // subtree
    else if (root->key < N) {
        int k = findMaxforN(root->right, N);
        if (k == -1)
            return root->key;
        else
            return k;
    }
 
    // If root's key is greater, return value
    // from left subtree.
    else if (root->key > N)
        return findMaxforN(root->left, N);   
}
 
// Driver code
int main()
{
    int N = 4;
 
    // creating following BST
    /*
                  5
               /   \
             2     12
           /  \    /  \
          1   3   9   21
                     /   \ 
                    19   25  */
    Node* root = insert(root, 25);
    insert(root, 2);
    insert(root, 1);
    insert(root, 3);
    insert(root, 12);
    insert(root, 9);
    insert(root, 21);
    insert(root, 19);
    insert(root, 25);
 
    printf("%d", findMaxforN(root, N));
 
    return 0;
}

Java




// Java code to find the largest value smaller
// than or equal to N
class GfG {
 
static class Node {
    int key;
    Node left, right;
}
 
// To create new BST Node
static Node newNode(int item)
{
    Node temp = new Node();
    temp.key = item;
    temp.left = null;
    temp.right = null;
    return temp;
}
 
// To insert a new node in BST
static Node insert(Node node, int key)
{
    // if tree is empty return new node
    if (node == null)
        return newNode(key);
 
    // if key is less then or greater then
    // node value then 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;
}
 
// function to find max value less then N
static int findMaxforN(Node root, int N)
{
    // Base cases
    if (root == null)
        return -1;
    if (root.key == N)
        return N;
 
    // If root's value is smaller, try in right
    // subtree
    else if (root.key < N) {
        int k = findMaxforN(root.right, N);
        if (k == -1)
            return root.key;
        else
            return k;
    }
 
    // If root's key is greater, return value
    // from left subtree.
    else if (root.key > N)
        return findMaxforN(root.left, N);
    return -1;
}
 
// Driver code
public static void main(String[] args)
{
    int N = 4;
 
    // creating following BST
    /*
                5
            / \
            2     12
        / \ / \
        1 3 9 21
                    / \
                    19 25 */
    Node root = null;
    root = insert(root, 25);
    insert(root, 2);
    insert(root, 1);
    insert(root, 3);
    insert(root, 12);
    insert(root, 9);
    insert(root, 21);
    insert(root, 19);
    insert(root, 25);
 
    System.out.println(findMaxforN(root, N));
}
}

Python3




# Python3 code to find the largest
# value smaller than or equal to N
class newNode:
 
    # Constructor to create a new node
    def __init__(self, data):
        self.key = data
        self.left = None
        self.right = None
 
# To insert a new node in BST
def insert(node, key):
     
    # if tree is empty return new node
    if node == None:
        return newNode(key)
 
    # if key is less then or greater then
    # node value then 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
 
# function to find max value less then N
def findMaxforN(root, N):
     
    # Base cases
    if root == None:
        return -1
    if root.key == N:
        return N
 
    # If root's value is smaller, try in
    # right subtree
    elif root.key < N:
        k = findMaxforN(root.right, N)
        if k == -1:
            return root.key
        else:
            return k
 
    # If root's key is greater, return
    # value from left subtree.
    elif root.key > N:
        return findMaxforN(root.left, N)
 
# Driver code
if __name__ == '__main__':
    N = 4
 
    # creating following BST
    #
    #             5
    #         / \
    #         2     12
    #     / \ / \
    #     1 3 9 21
    #                 / \
    #             19 25
    root = None
    root = insert(root, 25)
    insert(root, 2)
    insert(root, 1)
    insert(root, 3)
    insert(root, 12)
    insert(root, 9)
    insert(root, 21)
    insert(root, 19)
    insert(root, 25)
    print(findMaxforN(root, N))
 
# This code is contributed by PranchalK

C#




// C# code to find the largest value
// smaller than or equal to N
using System;
 
class GFG
{
 
    class Node
    {
        public int key;
        public Node left, right;
    }
 
    // To create new BST Node
    static Node newNode(int item)
    {
        Node temp = new Node();
        temp.key = item;
        temp.left = null;
        temp.right = null;
        return temp;
    }
 
    // To insert a new node in BST
    static Node insert(Node node, int key)
    {
        // if tree is empty return new node
        if (node == null)
            return newNode(key);
 
        // if key is less then or greater then
        // node value then 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;
    }
 
    // function to find max value less then N
    static int findMaxforN(Node root, int N)
    {
        // Base cases
        if (root == null)
            return -1;
        if (root.key == N)
            return N;
 
        // If root's value is smaller,
        // try in right subtree
        else if (root.key < N)
        {
            int k = findMaxforN(root.right, N);
            if (k == -1)
                return root.key;
            else
                return k;
        }
 
        // If root's key is greater, return
        // value from left subtree.
        else if (root.key > N)
            return findMaxforN(root.left, N);
        return -1;
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        int N = 4;
 
        // creating following BST
        /*
                    5
                / \
                2 12
            / \ / \
            1 3 9 21
                        / \
                        19 25 */
        Node root = null;
        root = insert(root, 25);
        insert(root, 2);
        insert(root, 1);
        insert(root, 3);
        insert(root, 12);
        insert(root, 9);
        insert(root, 21);
        insert(root, 19);
        insert(root, 25);
 
        Console.WriteLine(findMaxforN(root, N));
    }
}
 
// This code is contributed 29AjayKumar

Javascript




<script>
// javascript code to find the largest value smaller
// than or equal to N
     class Node
     {
         constructor(){
        this.key = 0;
        this.left = null,this.right = null;
    }
    }
 
    // To create new BST Node
    function newNode(item)
    {
        var temp = new Node();
        temp.key = item;
        temp.left = null;
        temp.right = null;
        return temp;
    }
 
    // To insert a new node in BST
    function insert(node , key) {
        // if tree is empty return new node
        if (node == null)
            return newNode(key);
 
        // if key is less then or greater then
        // node value then 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;
    }
 
    // function to find max value less then N
    function findMaxforN(root , N)
    {
     
        // Base cases
        if (root == null)
            return -1;
        if (root.key == N)
            return N;
 
        // If root's value is smaller, try in right
        // subtree
        else if (root.key < N)
        {
            var k = findMaxforN(root.right, N);
            if (k == -1)
                return root.key;
            else
                return k;
        }
 
        // If root's key is greater, return value
        // from left subtree.
        else if (root.key > N)
            return findMaxforN(root.left, N);
        return -1;
    }
 
    // Driver code
     
        var N = 4;
 
        // creating following BST
        /*
         * 5
         / \
         2 12
        / \ / \
        1 3 9 21
             / \
            19 25
         */
var root = null;
        root = insert(root, 25);
        insert(root, 2);
        insert(root, 1);
        insert(root, 3);
        insert(root, 12);
        insert(root, 9);
        insert(root, 21);
        insert(root, 19);
        insert(root, 25);
 
        document.write(findMaxforN(root, N));
 
// This code is contributed by Rajput-Ji
</script>

Output: 

3

Time complexity: O(h) where h is height of BST.
Reference : 
https://www.careercup.com/question?id=5765237112307712

Iterative Solution 
Below is an iterative solution and it does not require extra space for recursion call stack.  

C++




// C++ code to find the largest value smaller
// than or equal to N
#include <bits/stdc++.h>
using namespace std;
 
struct Node {
    int key;
    Node *left, *right;
};
 
// To create new BST Node
Node* newNode(int item)
{
    Node* temp = new Node;
    temp->key = item;
    temp->left = temp->right = NULL;
    return temp;
}
 
// To insert a new node in BST
Node* insert(Node* node, int key)
{
    // if tree is empty return new node
    if (node == NULL)
        return newNode(key);
 
    // if key is less then or greater then
    // node value then 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;
}
 
// function to find max value less then N
void findMaxforN(Node* root, int N)
{
    // Start from root and keep looking for larger 
    while (root != NULL && root->right != NULL) {
 
        // If root is smaller go to right side
        if (N > root->key && N >= root->right->key)
            root = root->right;
 
        // If root is greater go to left side
        else if (N < root->key)
            root = root->left;
 
        else
            break;
    }
    if (root == NULL || root->key > N)
        cout << -1;
    else
        cout << root->key;
}
 
// Driver code
int main()
{
    int N = 50;
    Node* root = insert(root, 5);
    insert(root, 2);
    insert(root, 1);
    insert(root, 3);
    insert(root, 12);
    insert(root, 9);
    insert(root, 21);
    insert(root, 19);
    insert(root, 25);
 
    findMaxforN(root, N);
    return 0;
}

Python3




# Python3 code to find the largest value
# smaller than or equal to N
 
class newNode:
     
    # To create new BST Node
    def __init__(self, data):
         
        self.key = data
        self.left = None
        self.right = None
 
# To insert a new node in BST
def insert(node, key):
     
    # If tree is empty return new node
    if (node == None):
        return newNode(key)
 
    # If key is less then or greater then
    # node value then 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
 
# Function to find max value less then N
def findMaxforN(root, N):
     
    # Start from root and keep looking for larger 
    while (root != None and root.right != None):
         
        # If root is smaller go to right side
        if (N > root.key and N >= root.right.key):
            root = root.right
 
        # If root is greater go to left side
        elif (N < root.key):
            root = root.left
        else:
            break
         
    if (root == None or root.key > N):
        print(-1)
    else:
        print(root.key)
 
# Driver code
if __name__ == '__main__':
     
    N = 50
     
    root = None
    root = insert(root, 5)
    insert(root, 2)
    insert(root, 1)
    insert(root, 3)
    insert(root, 12)
    insert(root, 9)
    insert(root, 21)
    insert(root, 19)
    insert(root, 25)
 
    findMaxforN(root, N)
 
# This code is contributed by bgangwar59

Javascript




<script>
 
 
// JavaScript code to find the largest value smaller
// than or equal to N
 
class Node
{
    constructor(data)
    {
        this.key=data;
        this.left=this.right=null;
    }
}
 
function insert(node, key)
{
    // If tree is empty return new node
    if (node == null)
        return new Node(key)
  
    // If key is less then or greater then
    // node value then 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
}
 
function findMaxforN(root, N)
{
    // Start from root and keep looking for larger
    while (root != null && root.right != null)
    {    
        // If root is smaller go to right side
        if (N > root.key && N >= root.right.key)
            root = root.right
  
        // If root is greater go to left side
        else if (N < root.key)
            root = root.left
        else
            break
    }    
    if (root == null || root.key > N)
        document.write(-1)
    else
        document.write(root.key)
}
let N = 50;
let root = null;
root=insert(root, 2)
root=insert(root, 1)
root=insert(root, 3)
root=insert(root, 12)
root=insert(root, 9)
root=insert(root, 21)
root=insert(root, 19)
root=insert(root, 25)
findMaxforN(root, N)
     
 
 
// This code is contributed by rag2127
 
</script>

Output:

25

 
 

This article is contributed by Niteesh Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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