Maximum number of Perfect Numbers present in a subarray of size K

Namachila November 19, 2025

0 0 8 minutes read

Given an array arr[ ] consisting of N integers, the task is to determine the maximum number of perfect Numbers in any subarray of size K.

Examples:

Input: arr[ ] = {28, 2, 3, 6, 496, 99, 8128, 24}, K = 4
Output: 3
Explanation: The sub-array {6, 496, 99, 8128} has 3 perfect numbers which is maximum.

Input: arr[ ]= {1, 2, 3, 6}, K=2
Output: 1

Naive Approach: The approach is to generate all possible subarrays of size K and for each subarray, count the number of elements that are a Perfect Number. Print the maximum count obtained for any subarray.

Time Complexity: O(N*K)
Auxiliary Space: O(1)

Efficient Approach:
To optimize the above approach, convert the given array arr[ ] into a binary array where the i^th element is 1 if it is a Perfect Number. Otherwise, the i^th element is 0. Therefore, the problem reduces to finding the maximum sum subarray of size K in the binary array using the Sliding Window technique. Follow the steps below to solve the problem:

Traverse the array and for each element of the array arr[], check if it is a Perfect Number or not.
If arr[i] is a Perfect Number then convert arr[i] equal to 1. Otherwise, convert arr[i] equal to 0.
To check if a number is a perfect number or not:
1. Initialize a variable sum to store the sum of divisors.
2. Traverse every number in the range [1, arr[i] – 1] and check if it is a divisor of arr[i] or not. Add all the divisors.
3. If the sum of all the divisors is equal to arr[i], then the number is a perfect number. Otherwise, the number is not a Perfect Number.
Compute the sum of the first subarray of size K in the modified array.
Using the sliding window technique, find the maximum sum of a subarray from all possible subarrays of size K.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to check a number
// is Perfect Number or not
int isPerfect(int N)
{
    // Stores sum of divisors
    int sum = 1;
 
    // Find all divisors and add them
    for (int i = 2; i < sqrt(N); i++) 
    {
 
        if (N % i == 0) {
 
            if (i == N / i) 
            {
 
                sum += i;
            }
            else
            {
 
                sum += i + N / i;
            }
        }
    }
 
    // If sum of divisors
    // is equal to N
    if (sum == N && N != 1)
        return 1;
 
    return 0;
}
 
// Function to return maximum
// sum of a subarray of size K
int maxSum(int arr[], int N, int K)
{
    // If k is greater than N
    if (N < K) 
    {
 
        cout << "Invalid";
        return -1;
    }
 
    // Compute sum of first window of size K
    int res = 0;
    for (int i = 0; i < K; i++) 
    {
 
        res += arr[i];
    }
 
    // Compute sums of remaining windows by
    // removing first element of previous
    // window and adding last element of
    // current window
    int curr_sum = res;
    for (int i = K; i < N; i++) 
    {
 
        curr_sum += arr[i] - arr[i - K];
        res = max(res, curr_sum);
    }
 
    // return the answer
    return res;
}
 
// Function to find all the
// perfect numbers in the array
int max_PerfectNumbers(int arr[], int N, int K)
{
    // The given array is converted into binary array
    for (int i = 0; i < N; i++) 
    {
 
        arr[i] = isPerfect(arr[i]) ? 1 : 0;
    }
 
    return maxSum(arr, N, K);
}
 
// Driver Code
int main()
{
    int arr[] = { 28, 2, 3, 6, 496, 99, 8128, 24 };
    int K = 4;
    int N = sizeof(arr) / sizeof(arr[0]);
 
    cout << max_PerfectNumbers(arr, N, K);
 
    return 0;
}

Java

// Java program for the 
// above approach
import java.util.*;
class GFG{
 
// Function to check a number
// is Perfect Number or not
static int isPerfect(int N)
{
  // Stores sum of divisors
  int sum = 1;
 
  // Find all divisors and 
  // add them
  for (int i = 2; 
           i < Math.sqrt(N); i++) 
  {
    if (N % i == 0) 
    {
      if (i == N / i)
      {
        sum += i;
      }
      else
      {
        sum += i + N / i;
      }
    }
  }
 
  // If sum of divisors
  // is equal to N
  if (sum == N && N != 1)
    return 1;
   
  return 0;
}
 
// Function to return maximum
// sum of a subarray of size K
static int maxSum(int arr[], 
                  int N, int K)
{
  // If k is greater than N
  if (N < K) 
  {
    System.out.print("Invalid");
    return -1;
  }
 
  // Compute sum of first 
  // window of size K
  int res = 0;
   
  for (int i = 0; i < K; i++) 
  {
    res += arr[i];
  }
 
  // Compute sums of remaining windows by
  // removing first element of previous
  // window and adding last element of
  // current window
  int curr_sum = res;
   
  for (int i = K; i < N; i++) 
  {
    curr_sum += arr[i] - arr[i - K];
    res = Math.max(res, curr_sum);
  }
 
  // return the answer
  return res;
}
 
// Function to find all the
// perfect numbers in the array
static int max_PerfectNumbers(int arr[], 
                              int N, int K)
{
  // The given array is converted 
  // into binary array
  for (int i = 0; i < N; i++) 
  {
    arr[i] = isPerfect(arr[i]) == 
             1 ? 1 : 0;
  }
 
  return maxSum(arr, N, K);
}
 
// Driver Code
public static void main(String[] args)
{
  int arr[] = {28, 2, 3, 6, 496,
               99, 8128, 24};
  int K = 4;
  int N = arr.length;
  System.out.print(max_PerfectNumbers(arr, 
                                      N, K));
}
}
 
// This code is contributed by Rajput-Ji

Python3

# Python3 program for the above approach
 
# Function to check a number
# is Perfect Number or not
def isPerfect(N):
     
    # Stores sum of divisors
    sum = 1
 
    # Find all divisors and add them
    for i in range(2, N):
        if i * i > N:
            break
 
        if (N % i == 0):
            if (i == N // i):
                sum += i
            else:
                sum += i + N // i
 
    # If sum of divisors
    # is equal to N
    if (sum == N and N != 1):
        return 1
 
    return 0
 
# Function to return maximum
# sum of a subarray of size K
def maxSum(arr, N, K):
     
    # If k is greater than N
    if (N < K):
        print("Invalid")
        return -1
 
    # Compute sum of first 
    # window of size K
    res = 0
     
    for i in range(K):
        res += arr[i]
 
    # Compute sums of remaining windows by
    # removing first element of previous
    # window and adding last element of
    # current window
    curr_sum = res
     
    for i in range(K, N):
        curr_sum += arr[i] - arr[i - K]
        res = max(res, curr_sum)
         
    # print(res)
 
    # Return the answer
    return res
 
# Function to find all the
# perfect numbers in the array
def max_PerfectNumbers(arr, N, K):
     
    # The given array is converted
    # into binary array
    for i in range(N):
        if isPerfect(arr[i]):
            arr[i] = 1
        else:
            arr[i] = 0
 
    return maxSum(arr, N, K)
 
# Driver Code
if __name__ == '__main__':
     
    arr = [ 28, 2, 3, 6, 
            496, 99, 8128, 24 ]
    K = 4
    N = len(arr)
 
    print(max_PerfectNumbers(arr, N, K))
     
# This code is contributed by mohit kumar 29

C#

// C# program for the 
// above approach
using System;
class GFG{
 
// Function to check a number
// is Perfect Number or not
static int isPerfect(int N)
{
  // Stores sum of divisors
  int sum = 1;
 
  // Find all divisors and 
  // add them
  for (int i = 2; 
           i < Math.Sqrt(N); i++) 
  {
    if (N % i == 0) 
    {
      if (i == N / i)
      {
        sum += i;
      }
      else
      {
        sum += i + N / i;
      }
    }
  }
 
  // If sum of divisors
  // is equal to N
  if (sum == N && N != 1)
    return 1;
   
  return 0;
}
 
// Function to return maximum
// sum of a subarray of size K
static int maxSum(int []arr, 
                  int N, int K)
{
  // If k is greater than N
  if (N < K) 
  {
    Console.Write("Invalid");
    return -1;
  }
 
  // Compute sum of first 
  // window of size K
  int res = 0;
   
  for (int i = 0; i < K; i++) 
  {
    res += arr[i];
  }
 
  // Compute sums of remaining 
  // windows by removing first 
  // element of previous window 
  // and adding last element of
  // current window
  int curr_sum = res;
   
  for (int i = K; i < N; i++) 
  {
    curr_sum += arr[i] - arr[i - K];
    res = Math.Max(res, curr_sum);
  }
 
  // return the answer
  return res;
}
 
// Function to find all the
// perfect numbers in the array
static int max_PerfectNumbers(int []arr, 
                              int N, int K)
{
  // The given array is converted 
  // into binary array
  for (int i = 0; i < N; i++) 
  {
    arr[i] = isPerfect(arr[i]) == 
             1 ? 1 : 0;
  }
 
  return maxSum(arr, N, K);
}
 
// Driver Code
public static void Main(String[] args)
{
  int []arr = {28, 2, 3, 6, 496,
               99, 8128, 24};
  int K = 4;
  int N = arr.Length;
  Console.Write(max_PerfectNumbers(arr, 
                                   N, K));
}
}
 
// This code is contributed by Amit Katiyar

Javascript

<script>
 
// Javascript program for the above approach
 
// Function to check a number
// is Perfect Number or not
function isPerfect(N)
{
     
    // Stores sum of divisors
    let sum = 1;
     
    // Find all divisors and
    // add them
    for(let i = 2;
            i < Math.sqrt(N); i++)
    {
        if (N % i == 0)
        {
            if (i == N / i)
            {
                sum += i;
            }
            else
            {
                sum += i + N / i;
            }
        }
    }
     
    // If sum of divisors
    // is equal to N
    if (sum == N && N != 1)
        return 1;
     
    return 0;
}
  
// Function to return maximum
// sum of a subarray of size K
function maxSum(arr, N, K)
{
     
    // If k is greater than N
    if (N < K)
    {
        document.write("Invalid");
        return -1;
    }
     
    // Compute sum of first
    // window of size K
    let res = 0;
     
    for(let i = 0; i < K; i++)
    {
        res += arr[i];
    }
     
    // Compute sums of remaining windows by
    // removing first element of previous
    // window and adding last element of
    // current window
    let curr_sum = res;
     
    for(let i = K; i < N; i++)
    {
        curr_sum += arr[i] - arr[i - K];
        res = Math.max(res, curr_sum);
    }
     
    // Return the answer
    return res;
}
  
// Function to find all the
// perfect numbers in the array
function max_PerfectNumbers(arr, N, K)
{
     
    // The given array is converted
    // into binary array
    for(let i = 0; i < N; i++)
    {
        arr[i] = isPerfect(arr[i]) ==
                  1 ? 1 : 0;
    }
    return maxSum(arr, N, K);
}
 
// Driver Code
let arr = [ 28, 2, 3, 6, 496,
            99, 8128, 24 ];
let K = 4;
let N = arr.length;
 
document.write(max_PerfectNumbers(arr, N, K));
 
// This code is contributed by target_2    
 
</script>

Output:

Time Complexity: O( N * sqrt(N) )
Auxiliary Space: O(1)

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