Data Modelling & AI Data Structure & Algorithm

Second unique smallest value of given Binary Tree whose each node is minimum of its children

23 June 2025

0

Given a full binary tree where each node value is the same as the minimum value between its children, the task is to find the second minimum unique value of the tree.

Examples:

Input: tree:

Example of the problem statement

Output: 5
Explanation: All the unique values present in the tree are 2, 5 and 7.
The second smallest is 5.

Input: tree:

Another example of a problem statement

Output: -1
Explanation: All the numbers present in the tree are same.
There is no other value except 2, so second smallest is not possible.

Naive Approach: The basic approach to solve the problem is based on the idea:

Store all the unique values in an array and sort the array. Then find the second minimum from the array.

Follow the below step to understand the approach more clearly:

Traverse the tree and store all the values in an array.
Sort the array.
Iterate over the array and find the first array element which is not equal to the minimum element of the array (i.e. the one present at index 0). If no such element is present then return -1.

Below is the implementation of the above approach.

C++

// C++ code to implement the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Structure of a tree node
struct Node {
    int data;
    struct Node* left;
    struct Node* right;
    Node(int val)
    {
        data = val;
        left = NULL;
        right = NULL;
    }
};
 
// Initialize a vector
vector<int> ans;
 
// Traversing the tree by using inorder
// traversal
void inorderTraversal(Node* root)
{
    if (root != NULL) {
        inorderTraversal(root->left);
        ans.push_back(root->data);
        inorderTraversal(root->right);
    }
}
 
// Function to find the second minimum element
int findSecondMinimumValue(Node* root)
{
    inorderTraversal(root);
 
    // Sorting the array element
    sort(ans.begin(), ans.end());
 
    // Traversing and array and checking
    // the second minimum element
    for (int i = 0; i < ans.size() - 1; i++)
        if (ans[i] != ans[i + 1])
            return ans[i + 1];
    return -1;
}
 
// Driver code
int main()
{
    // Creating the tree
    // 2
    //        / \
    // 2   5
    //          / \
    // 5   7
    struct Node* root = new Node(2);
    root->left = new Node(2);
    root->right = new Node(5);
    root->right->left = new Node(5);
    root->right->right = new Node(7);
 
    // Function call
    int SecondMinimumValue
        = findSecondMinimumValue(root);
    cout << SecondMinimumValue << endl;
    return 0;
}

Java

// JAVA code to implement the above approach
import java.util.*;
 
class GFG 
{
 
  // Structure of a tree node
  public static class Node {
    int data;
    Node left;
    Node right;
    Node(int val) { this.data = val; }
  }
 
  // Initialize a vector
  public static ArrayList<Integer> ans
    = new ArrayList<Integer>();
 
  // Traversing the tree by using inorder
  // traversal
  public static void inorderTraversal(Node root)
  {
    if (root != null) {
      inorderTraversal(root.left);
      ans.add(root.data);
      inorderTraversal(root.right);
    }
  }
 
  // Function to find the second minimum element
  public static int findSecondMinimumValue(Node root)
  {
    inorderTraversal(root);
 
    // Sorting the array element
    Collections.sort(ans);
 
    // Traversing and array and checking
    // the second minimum element
    for (int i = 0; i < ans.size() - 1; i++)
      if (ans.get(i) != ans.get(i + 1))
        return ans.get(i + 1);
    return -1;
  }
 
  // Driver code
  public static void main(String[] args)
  {
    // Creating the tree
    // 2
    //        / \
    // 2   5
    //          / \
    // 5   7
    Node root = new Node(2);
    root.left = new Node(2);
    root.right = new Node(5);
    root.right.left = new Node(5);
    root.right.right = new Node(7);
 
    // Function call
    int SecondMinimumValue
      = findSecondMinimumValue(root);
    System.out.println(SecondMinimumValue);
  }
}
 
// This code is contributed by Taranpreet

Python3

# Python3 code for the above approach
 
# class to create a tree node
class Node:
    def __init__(self, d):
        self.data = d
        self.left = None
        self.right = None
             
# Initialize a list
ans = []
 
# Traversing the tree by using inorder
# traversal
def inorderTraversal(root) :
    if (root != None) :
        inorderTraversal(root.left)
        ans.append(root.data)
        inorderTraversal(root.right)
             
         
 
    # Function to find the second minimum element
def findSecondMinimumValue(root) :
    inorderTraversal(root)
 
    # Sorting the array element
    ans.sort()
 
    # Traversing and array and checking
    # the second minimum element
    for i in range(0,len(ans)-1) :
        if (ans[i] != ans[i + 1]) :
            return ans[i + 1]
    return -1
 
# Driver Code
if __name__ == '__main__':
         
    # Driver code
 
    # Creating the tree
    #   2
    #  / \
    # 2   5
    #    / \
    #   5   7
    root = Node(2)
    root.left = Node(2)
    root.right = Node(5)
    root.right.left = Node(5)
    root.right.right = Node(7)
 
    # Function call
    SecondMinimumValue = findSecondMinimumValue(root)
    print(SecondMinimumValue)
 
    # This code is contributed by jana_sayantan.

Javascript

<script>
       // JavaScript code for the above approach
 
       // Structure of a tree node
       class Node {
 
           constructor(val) {
               this.data = val;
               this.left = null;
               this.right = null;
           }
       };
 
       // Initialize a vector
       let ans = [];
 
       // Traversing the tree by using inorder
       // traversal
       function inorderTraversal(root) {
           if (root != null) {
               inorderTraversal(root.left);
               ans.push(root.data);
               inorderTraversal(root.right);
           }
       }
 
       // Function to find the second minimum element
       function findSecondMinimumValue(root) {
           inorderTraversal(root);
 
           // Sorting the array element
           ans.sort();
 
           // Traversing and array and checking
           // the second minimum element
           for (let i = 0; i < ans.length - 1; i++)
               if (ans[i] != ans[i + 1])
                   return ans[i + 1];
           return -1;
       }
 
       // Driver code
 
       // Creating the tree
       // 2
       //        / \
       // 2   5
       //          / \
       // 5   7
       let root = new Node(2);
       root.left = new Node(2);
       root.right = new Node(5);
       root.right.left = new Node(5);
       root.right.right = new Node(7);
 
       // Function call
       let SecondMinimumValue
           = findSecondMinimumValue(root);
       document.write(SecondMinimumValue + '<br>');
 
   // This code is contributed by Potta Lokesh
   </script>

C#

// C# code to implement the above approach
using System;
using System.Collections.Generic;
 
public class GFG 
{
 
  // Structure of a tree node
  public class Node {
    public int data;
    public Node left;
    public Node right;
    public Node(int val) { this.data = val; }
  }
 
  // Initialize a vector
  public static List<int> ans
    = new List<int>();
 
  // Traversing the tree by using inorder
  // traversal
  public static void inorderTraversal(Node root)
  {
    if (root != null) {
      inorderTraversal(root.left);
      ans.Add(root.data);
      inorderTraversal(root.right);
    }
  }
 
  // Function to find the second minimum element
  public static int findSecondMinimumValue(Node root)
  {
    inorderTraversal(root);
 
    // Sorting the array element
    ans.Sort();
 
    // Traversing and array and checking
    // the second minimum element
    for (int i = 0; i < ans.Count - 1; i++)
      if (ans[i] != ans[i + 1])
        return ans[i + 1];
    return -1;
  }
 
  // Driver code
  public static void Main(String[] args)
  {
    // Creating the tree
    // 2
    //        / \
    // 2   5
    //          / \
    // 5   7
    Node root = new Node(2);
    root.left = new Node(2);
    root.right = new Node(5);
    root.right.left = new Node(5);
    root.right.right = new Node(7);
 
    // Function call
    int SecondMinimumValue
      = findSecondMinimumValue(root);
    Console.WriteLine(SecondMinimumValue);
  }
}
 
 
// This code contributed by shikhasingrajput

Output

Time Complexity: O(N * logN)
Auxiliary Space: O(N)

Efficient Approach: The problem can be solved efficiently by using the below observation:

The root node of the tree contains the minimum among all the nodes.
If the value of all the nodes with minimum value is replaced by some other arbitrary numbers out of the range of tree values and the tree property is maintained then the root node will have the current minimum value which is actually the second minimum of the given tree.

The above observation can be implemented using recursion. Follow the approach mentioned below to implement the idea of the above observation:

Use a recursive function to traverse the tree and implement the observation.
In each recursion for any node:
- Find which of its children has the same value as the root and call the next recursion for that child.
- If the value of the current node is the same as the root and does not have any children, or both children have same value replace its value with -1.
- If the current node has children, replace it with the minimum of its children while returning from the recursive function. (This will replace the value with the second minimum as the minimum is replaced with -1 and -1 is not being considered for checking the minimum).
In this way, while the traversal is complete, the tree root holds the current minimum of the tree which is actually the second minimum of the original tree.
Return the value of the root.

Below is the code of the above approach:

C++

// C++ program for above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Structure of a tree node
struct Node {
    int data;
    struct Node* left;
    struct Node* right;
    Node(int val)
    {
        data = val;
        left = NULL;
        right = NULL;
    }
};
 
// Function to find the minimum value
int findSecondMinimumValue(Node* root)
{
    // When the root is null
    if (!root)
        return -1;
 
    // Base Condition
    // When we reach the Leaf node then
    // in that case we have to return -1
    // as per the algorithm stated in
    // the above statement
    if (!root->left && !root->right)
        return -1;
 
    // Storing the Node value of the left
    // child of the Node
    auto left = root->left->data;
 
    // Storing the Node value of the right
    // child of the Node
    auto right = root->right->data;
 
    // Call the function recursively to the
    // left sub-part of the tree if the value
    // of the node value matches with its left
    // child node value
    if (root->data == root->left->data)
        left
            = findSecondMinimumValue(root->left);
 
    // Call the function recursively to the
    // right sub-part of the tree if the
    // value of the node value matches with
    // its right child node value
    if (root->data == root->right->data)
        right
            = findSecondMinimumValue(root->right);
 
    // In case if both the left and right
    // child value is not equal to -1 then
    // in that case return the minimum of
    // them to  its parent
    if (left != -1 && right != -1)
        return min(left, right);
 
    // In case if the left child's value is
    // not equal to -1 BUT its right child's
    // value is equal to -1 then in that case
    // send the left child value to its
    // parent node.
    else if (left != -1)
        return left;
 
    // In case if the right child's value is
    // not equal to -1 BUT its left child's
    // value is equal to -1 then in that case
    // send the right child value to its
    // parent node.
    else
        return right;
}
 
// Driver code
int main()
{
    // Creating the root node
    /*         2
              / \
             2   5
                / \
               5   7 */
    struct Node* root = new Node(2);
    root->left = new Node(2);
    root->right = new Node(5);
    root->right->left = new Node(5);
    root->right->right = new Node(7);
 
    int SecondMinimumValue
        = findSecondMinimumValue(root);
    cout << SecondMinimumValue << endl;
    return 0;
}

C

// C program for above approach
 
#include <stdio.h>
#include <stdlib.h>
 
// Structure of a tree node
typedef struct Node {
    int data;
    struct Node* left;
    struct Node* right;
} Node;
 
Node* newNode(int data)
{
    Node* new_node = (Node*)malloc(sizeof(Node));
    new_node->data = data;
    new_node->left = new_node->right = NULL;
    return new_node;
}
 
// Find minimum between two numbers.
int min(int num1, int num2)
{
  return (num1 > num2) ? num2 : num1;
}
 
// Function to find the minimum value
int findSecondMinimumValue(Node* root)
{
    // When the root is null
    if (!root)
        return -1;
 
    // Base Condition
    // When we reach the Leaf node then
    // in that case we have to return -1
    // as per the algorithm stated in
    // the above statement
    if (!root->left && !root->right)
        return -1;
 
    // Storing the Node value of the left
    // child of the Node
    int left = root->left->data;
 
    // Storing the Node value of the right
    // child of the Node
    int right = root->right->data;
 
    // Call the function recursively to the
    // left sub-part of the tree if the value
    // of the node value matches with its left
    // child node value
    if (root->data == root->left->data)
        left = findSecondMinimumValue(root->left);
 
    // Call the function recursively to the
    // right sub-part of the tree if the
    // value of the node value matches with
    // its right child node value
    if (root->data == root->right->data)
        right = findSecondMinimumValue(root->right);
 
    // In case if both the left and right
    // child value is not equal to -1 then
    // in that case return the minimum of
    // them to the its parent
    if (left != -1 && right != -1)
        return min(left, right);
 
    // In case if the left child's value is
    // not equal to -1 BUT its right child's
    // value is equal to -1 then in that case
    // send the left child value to its
    // parent node.
    else if (left != -1)
        return left;
 
    // In case if the right child's value is
    // not equal to -1 BUT its left child's
    // value is equal to -1 then in that case
    // send the right child value to its
    // parent node.
    else
        return right;
}
 
// Driver code
int main()
{
    // Creating the root node
    /*         2
              / \
             2   5
                / \
               5   7 */
    Node* root = newNode(2);
    root->left = newNode(2);
    root->right = newNode(5);
    root->right->left = newNode(5);
    root->right->right = newNode(7);
 
    int SecondMinimumValue = findSecondMinimumValue(root);
    printf("%d\n", SecondMinimumValue);
    return 0;
}
 
// This code is contributed by Sania Kumari Gupta

Java

/*package whatever //do not write package name here */
import java.io.*;
import java.util.*;
 
class GFG {
 
  static class Node {
    int data;
    Node left;
    Node right;
    Node(int val)
    {
      data = val;
      left = null;
      right = null;
    }
  }
 
  // Function to find the minimum value
  static int findSecondMinimumValue(Node root)
  {
    // When the root is null
    if (root==null)
      return -1;
 
    // Base Condition
    // When we reach the Leaf node then
    // in that case we have to return -1
    // as per the algorithm stated in
    // the above statement
    if (root.left==null && root.right==null)
      return -1;
 
    // Storing the Node value of the left
    // child of the Node
    int left = root.left.data;
 
    // Storing the Node value of the right
    // child of the Node
    int right = root.right.data;
 
    // Call the function recursively to the
    // left sub-part of the tree if the value
    // of the node value matches with its left
    // child node value
    if (root.data == root.left.data)
      left = findSecondMinimumValue(root.left);
 
    // Call the function recursively to the
    // right sub-part of the tree if the
    // value of the node value matches with
    // its right child node value
    if (root.data == root.right.data)
      right = findSecondMinimumValue(root.right);
 
    // In case if both the left and right
    // child value is not equal to -1 then
    // in that case return the minimum of
    // them to the its parent
    if (left != -1 && right != -1)
      return Math.min(left, right);
 
    // In case if the left child's value is
    // not equal to -1 BUT its right child's
    // value is equal to -1 then in that case
    // send the left child value to its
    // parent node.
    else if (left != -1)
      return left;
 
    // In case if the right child's value is
    // not equal to -1 BUT its left child's
    // value is equal to -1 then in that case
    // send the right child value to its
    // parent node.
    else
      return right;
  }
 
 
  public static void main (String[] args) {
 
    // Creating the root node
    /*         2
              / \
             2   5
                / \
               5   7 */
    Node root = new Node(2);
    root.left = new Node(2);
    root.right = new Node(5);
    root.right.left = new Node(5);
    root.right.right = new Node(7);
 
    int SecondMinimumValue = findSecondMinimumValue(root);
 
    System.out.println(SecondMinimumValue);
  }
}
 
// This code is contributed by aadityaburujwale.

Python3

# Python program to implement above approach
 
# Structure of a tree node
class Node:
    def __init__(self,val):
        self.data = val
        self.left = None
        self.right = None
     
 
# Function to find the minimum value
def findSecondMinimumValue(root):
    # When the root is None
    if (root == None):
        return -1
 
    # Base Condition
    # When we reach the Leaf node then
    # in that case we have to return -1
    # as per the algorithm stated in
    # the above statement
    if (root.left == None and root.right == None):
        return -1
 
    # Storing the Node value of the left
    # child of the Node
    left = root.left.data
 
    # Storing the Node value of the right
    # child of the Node
    right = root.right.data
 
    # Call the function recursively to the
    # left sub-part of the tree if the value
    # of the node value matches with its left
    # child node value
    if (root.data == root.left.data):
        left = findSecondMinimumValue(root.left)
 
    # Call the function recursively to the
    # right sub-part of the tree if the
    # value of the node value matches with
    # its right child node value
    if (root.data == root.right.data):
        right = findSecondMinimumValue(root.right)
 
    # In case if both the left and right
    # child value is not equal to -1 then
    # in that case return the minimum of
    # them to the its parent
    if (left != -1 and right != -1):
        return min(left, right)
 
    # In case if the left child's value is
    # not equal to -1 BUT its right child's
    # value is equal to -1 then in that case
    # send the left child value to its
    # parent node.
    elif (left != -1):
        return left
 
    # In case if the right child's value is
    # not equal to -1 BUT its left child's
    # value is equal to -1 then in that case
    # send the right child value to its
    # parent node.
    else:
        return right
 
# Driver code
 
root = Node(2)
root.left = Node(2)
root.right = Node(5)
root.right.left = Node(5)
root.right.right = Node(7)
         
SecondMinimumValue = findSecondMinimumValue(root)
print(SecondMinimumValue)
 
# This code is contributed by shinjanpatra

C#

// C# program for above approach
using System;
 
public class GFG {
 
  class Node {
    public int data;
    public Node left;
    public Node right;
    public Node(int val)
    {
      data = val;
      left = null;
      right = null;
    }
  }
 
  // Function to find the minimum value
  static int findSecondMinimumValue(Node root)
  {
    // When the root is null
    if (root == null)
      return -1;
 
    // Base Condition
    // When we reach the Leaf node then
    // in that case we have to return -1
    // as per the algorithm stated in
    // the above statement
    if (root.left == null && root.right == null)
      return -1;
 
    // Storing the Node value of the left
    // child of the Node
    int left = root.left.data;
 
    // Storing the Node value of the right
    // child of the Node
    int right = root.right.data;
 
    // Call the function recursively to the
    // left sub-part of the tree if the value
    // of the node value matches with its left
    // child node value
    if (root.data == root.left.data)
      left = findSecondMinimumValue(root.left);
 
    // Call the function recursively to the
    // right sub-part of the tree if the
    // value of the node value matches with
    // its right child node value
    if (root.data == root.right.data)
      right = findSecondMinimumValue(root.right);
 
    // In case if both the left and right
    // child value is not equal to -1 then
    // in that case return the minimum of
    // them to the its parent
    if (left != -1 && right != -1)
      return Math.Min(left, right);
 
    // In case if the left child's value is
    // not equal to -1 BUT its right child's
    // value is equal to -1 then in that case
    // send the left child value to its
    // parent node.
    else if (left != -1)
      return left;
 
    // In case if the right child's value is
    // not equal to -1 BUT its left child's
    // value is equal to -1 then in that case
    // send the right child value to its
    // parent node.
    else
      return right;
  }
 
  static public void Main()
  {
 
    // Code
    // Creating the root node
    /*         2
                  / \
                 2   5
                    / \
                   5   7 */
    Node root = new Node(2);
    root.left = new Node(2);
    root.right = new Node(5);
    root.right.left = new Node(5);
    root.right.right = new Node(7);
 
    int SecondMinimumValue
      = findSecondMinimumValue(root);
 
    Console.WriteLine(SecondMinimumValue);
  }
}
 
// This code is contributed by lokeshmvs21.

Javascript

<script>
 
// JavaScript program to implement above approach
 
// Structure of a tree node
class Node {    
    constructor(val)
    {
        this.data = val;
        this.left = null;
        this.right = null;
    }
};
 
// Function to find the minimum value
function findSecondMinimumValue(root)
{
    // When the root is null
    if (!root)
        return -1;
 
    // Base Condition
    // When we reach the Leaf node then
    // in that case we have to return -1
    // as per the algorithm stated in
    // the above statement
    if (!root.left && !root.right)
        return -1;
 
    // Storing the Node value of the left
    // child of the Node
    let left = root.left.data;
 
    // Storing the Node value of the right
    // child of the Node
    let right = root.right.data;
 
    // Call the function recursively to the
    // left sub-part of the tree if the value
    // of the node value matches with its left
    // child node value
    if (root.data == root.left.data)
        left = findSecondMinimumValue(root.left);
 
    // Call the function recursively to the
    // right sub-part of the tree if the
    // value of the node value matches with
    // its right child node value
    if (root.data == root.right.data)
        right = findSecondMinimumValue(root.right);
 
    // In case if both the left and right
    // child value is not equal to -1 then
    // in that case return the minimum of
    // them to the its parent
    if (left != -1 && right != -1)
        return Math.min(left, right);
 
    // In case if the left child's value is
    // not equal to -1 BUT its right child's
    // value is equal to -1 then in that case
    // send the left child value to its
    // parent node.
    else if (left != -1)
        return left;
 
    // In case if the right child's value is
    // not equal to -1 BUT its left child's
    // value is equal to -1 then in that case
    // send the right child value to its
    // parent node.
    else
        return right;
}
 
// Driver code
 
let root = new Node(2);
root.left = new Node(2);
root.right = new Node(5);
root.right.left = new Node(5);
root.right.right = new Node(7);
         
let SecondMinimumValue = findSecondMinimumValue(root);
document.write(SecondMinimumValue,"</br>");
 
// This code is contributed by shinjanpatra
 
</script>

Output

Time Complexity: O(H) where H is the height of the tree
Auxiliary Space: O(1)

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Second unique smallest value of given Binary Tree whose each node is minimum of its children

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