# Length of Longest Perfect number Subsequence in an Array

• Last Updated : 03 Jun, 2021

Given an array arr[] containing non-negative integers of length N, the task is to print the length of the longest subsequence of the Perfect number in the array.

A number is a perfect number if it is equal to the sum of its proper divisors, that is, the sum of its positive divisors excluding the number itself.

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Examples:

Input: arr[] = { 3, 6, 11, 2, 28, 21, 8128 }
Output:
Explanation:
The longest perfect number subsequence is {6, 28, 8128} and hence the answer is 3.

Input:arr[] = { 6, 4, 10, 13, 9, 25 }
Output:
Explanation:
The longest perfect number subsequence is {6} and hence the answer is 1.

Approach:
To solve the problem mentioned above, follow the steps given below:

• Traverse the given array and for each element in the array, check if it is a perfect number or not.
• If the element is a perfect number, it will be in the Longest Perfect number Subsequence. Hence, increment the required length of the Longest Perfect number Subsequence by 1

Below is the implementation of the above approach:

## C++

 `// C++ program to find the length of``// Longest Perfect number Subsequence in an Array` `#include ``using` `namespace` `std;` `// Function to check if``// the number is a Perfect number``bool` `isPerfect(``long` `long` `int` `n)``{``    ``// To store sum of divisors``    ``long` `long` `int` `sum = 1;` `    ``// Find all divisors and add them``    ``for` `(``long` `long` `int` `i = 2; i * i <= n; i++) {``        ``if` `(n % i == 0) {``            ``if` `(i * i != n)``                ``sum = sum + i + n / i;``            ``else``                ``sum = sum + i;``        ``}``    ``}``    ``// Check if sum of divisors is equal to``    ``// n, then n is a perfect number``    ``if` `(sum == n && n != 1)``        ``return` `true``;` `    ``return` `false``;``}` `// Function to find the longest subsequence``// which contain all Perfect numbers``int` `longestPerfectSubsequence(``int` `arr[], ``int` `n)``{``    ``int` `answer = 0;` `    ``// Find the length of longest``    ``// Perfect number subsequence``    ``for` `(``int` `i = 0; i < n; i++) {``        ``if` `(isPerfect(arr[i]))``            ``answer++;``    ``}` `    ``return` `answer;``}` `// Driver code``int` `main()``{``    ``int` `arr[] = { 3, 6, 11, 2, 28, 21, 8128 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``cout << longestPerfectSubsequence(arr, n) << endl;` `    ``return` `0;``}`

## Java

 `// Java program to find the length of``// longest perfect number subsequence``// in an array``class` `GFG {``    ` `// Function to check if the``// number is a perfect number``static` `boolean` `isPerfect(``long` `n)``{``    ` `    ``// To store sum of divisors``    ``long` `sum = ``1``;``    ` `    ``// Find all divisors and add them``    ``for``(``long` `i = ``2``; i * i <= n; i++)``    ``{``       ``if` `(n % i == ``0``)``       ``{``           ``if` `(i * i != n)``               ``sum = sum + i + n / i;``           ``else``               ``sum = sum + i;``       ``}``    ``}``    ` `    ``// Check if sum of divisors is equal ``    ``// to n, then n is a perfect number``    ``if` `(sum == n && n != ``1``)``    ``{``        ``return` `true``;``    ``}``    ``return` `false``;``}``    ` `// Function to find the longest subsequence``// which contain all Perfect numbers``static` `int` `longestPerfectSubsequence(``int` `arr[],``                                     ``int` `n)``{``    ``int` `answer = ``0``;``    ` `    ``// Find the length of longest``    ``// perfect number subsequence``    ``for``(``int` `i = ``0``; i < n; i++)``    ``{``       ``if` `(isPerfect(arr[i]) == ``true``)``           ``answer++;``    ``}``    ``return` `answer;``}``    ` `// Driver code``public` `static` `void` `main (String[] args)``{``    ``int` `arr[] = { ``3``, ``6``, ``11``, ``2``, ``28``, ``21``, ``8128` `};``    ``int` `n = arr.length;``    ` `    ``System.out.println(longestPerfectSubsequence(arr, n));``}``}` `// This code is contributed by AnkitRai01`

## Python3

 `# Python3 program to find the length of``# Longest Perfect number Subsequence in an Array`  `# Function to check if``# the number is Perfect number``def` `isPerfect( n ):``    ` `    ``# To store sum of divisors``    ``sum` `=` `1``    ` `    ``# Find all divisors and add them``    ``i ``=` `2``    ``while` `i ``*` `i <``=` `n:``        ``if` `n ``%` `i ``=``=` `0``:``            ``sum` `=` `sum` `+` `i ``+` `n ``/` `i``        ``i ``+``=` `1``    ` `    ``# Check if sum of divisors is equal to``    ``# n, then n is a perfect number``    ` `    ``return` `(``True` `if` `sum` `=``=` `n ``and` `n !``=` `1` `else` `False``)` `# Function to find the longest subsequence``# which contain all Perfect numbers``def` `longestPerfectSubsequence( arr, n):``    ` `    ``answer ``=` `0``    ` `    ``# Find the length of longest``    ``# Perfect number subsequence``    ``for` `i ``in` `range` `(n):``        ``if` `(isPerfect(arr[i])):``            ``answer ``+``=` `1``    ` `    ``return` `answer` `# Driver code``if` `__name__ ``=``=` `"__main__"``:``    ``arr ``=` `[ ``3``, ``6``, ``11``, ``2``, ``28``, ``21``, ``8128` `]``    ``n ``=` `len``(arr)``    ` `    ``print` `(longestPerfectSubsequence(arr, n))`

## C#

 `// C# program to find the length of``// longest perfect number subsequence``// in an array``using` `System;` `class` `GFG {``    ` `// Function to check if the``// number is a perfect number``static` `bool` `isPerfect(``long` `n)``{``        ` `    ``// To store sum of divisors``    ``long` `sum = 1;``        ` `    ``// Find all divisors and add them``    ``for``(``long` `i = 2; i * i <= n; i++)``    ``{``       ``if` `(n % i == 0)``       ``{``           ``if` `(i * i != n)``               ``sum = sum + i + n / i;``           ``else``               ``sum = sum + i;``       ``}``    ``}``    ` `    ``// Check if sum of divisors is equal``    ``// to n, then n is a perfect number``    ``if` `(sum == n && n != 1)``    ``{``        ``return` `true``;``    ``}``    ``return` `false``;``}``        ` `// Function to find the longest subsequence``// which contain all perfect numbers``static` `int` `longestPerfectSubsequence(``int` `[]arr,``                                     ``int` `n)``{``    ``int` `answer = 0;``        ` `    ``// Find the length of longest``    ``// perfect number subsequence``    ``for``(``int` `i = 0; i < n; i++)``    ``{``       ``if` `(isPerfect(arr[i]) == ``true``)``           ``answer++;``    ``}``    ``return` `answer;``}``        ` `// Driver code``public` `static` `void` `Main (``string``[] args)``{``    ``int` `[]arr = { 3, 6, 11, 2, 28, 21, 8128 };``    ``int` `n = arr.Length;``        ` `    ``Console.WriteLine(longestPerfectSubsequence(arr, n));``}``}` `// This code is contributed by AnkitRai01`

## Javascript

 ``
Output:
`3`

Time Complexity: O(N×√N)
Auxiliary Space Complexity: O(1)

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