Find all possible GCDs of every subsequences of given Array

Given an array arr[] consisting of N positive integers, the task is to find all the possible distinct Greatest Common Divisors(GCDs) among all the non-empty subsequences of the array arr[].

Examples:

Input: arr[] = {3, 4, 8}
Output: 1 3 4 8
Explanation:
The non-empty subsequences possible are {3}, {4}, {8}, {3, 4}, {4, 8}, {3, 8}, {3, 4, 8} and their corresponding GCDs are 3, 4, 8, 1, 4, 1, 1. 
Therefore, print all the GCDs as {1, 3, 4, 8}.

Input: arr[] = {3, 8, 9, 4, 13, 45, 6}
Output: 1 2 3 4 6 8 9 13 45

Naive Approach: The simplest approach to solve the given problem is to generate all possible subsequences of the given array and store all the GCDs of the subsequence in a set. After checking for all the subsequences, print the element stores in the set as all possible GCDs that can be formed.

Time Complexity: O(log M*2^N), where M is the maximum element of the array.
Auxiliary Space: O(1)

Efficient Approach: The above approach can also be optimized using Greedy Approach by observing the fact that the GCD of any subsequence lies over the range [1, M] where M is the maximum element of the array. Therefore, the idea is to iterate over then range [1, M] and if any element in the range is a factor of an array element then print the current element as one of the resultant GCD. Follow the steps below to solve the problem:

• Store all the array elements in the HashSet, say s.
• Iterate over the range [1, M] using the variable i and perform the following steps:
  • Iterate through all the multiples of i, if there exists any multiple which is present in the HashSet, then print the current element i as one of the possible GCD.

Below is an implementation of the above approach:

// C++ program for the above approach

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Function to find the different GCDs of
// the subsequence of the given array
void findGCDsSubsequence(vector<int> arr)
{
    
    // Stores all the possible GCDs
    vector<int> ans;

    // Stores all array element in set
    set<int> s;
    for(int i : arr)
        s.insert(i);
        
    int M = *max_element(arr.begin(), arr.end());
    
    // Iterate over the range [1, M]
    for(int i = 1; i <= M; i++) 
    {
        int gcd = 0;

        // Check if i can be the GCD of
        // any subsequence
        for(int j = i; j < M + 1; j += i) 
        {
            if (s.find(j) != s.end())
                gcd = __gcd(gcd, j);
        }
        if (gcd == i)
            ans.push_back(i);
    }
    for(int i = 0; i < ans.size(); i++)
        cout << ans[i] << " ";
}

// Driver Code
int main()
{
    int N = 7;
    vector<int> arr = { 3, 4, 8 };

    // Function Call
    findGCDsSubsequence(arr);
    return 0;
}

// This code is contributed by parthagarwal1962000

// Java program for the above approach
import java.util.*; 
public class GFG {
static int gcd1(int a, int b)
{
    return b == 0 ? a : gcd1(b, a % b);
}

// Function to find the different GCDs of
// the subsequence of the given array
static void findGCDsSubsequence(ArrayList<Integer> arr)
{
    
    // Stores all the possible GCDs
    ArrayList<Integer> ans = new ArrayList<Integer>();

    // Stores all array element in set
    HashSet<Integer> s = new HashSet<Integer>();
    for(int i : arr)
        s.add(i);
        
    int M = Integer.MIN_VALUE;
    for(int i : arr)
    {
        if (i > M)
            M = i;
    }

    // Iterate over the range [1, M]
    for(int i = 1; i <= M; i++) 
    {
        int gcd = 0;

        // Check if i can be the GCD of
        // any subsequence
        for(int j = i; j < M + 1; j += i) 
        {
            if (s.contains(j))
                gcd = gcd1(gcd, j);
        }
        if (gcd == i)
            ans.add(i);
    }
    for(int i = 0; i < ans.size(); i++)
       System.out.print(ans.get(i) + " ");
}

// Driver Code
 public static void main(String args[]) 
{
    ArrayList<Integer> arr = new ArrayList<Integer>();
    arr.add(3);
    arr.add(4);
    arr.add(8);

    // Function Call
    findGCDsSubsequence(arr);
}
}
// This code is contributed by SoumikMondal

# Python3 program for the above approach
import math

# Function to find the different GCDs of
# the subsequence of the given array
def findGCDsSubsequence(nums):

        # Stores all the possible GCDs
    Ans = []

    # Stores all array element in set
    s = set(nums)

    # Find the maximum array element
    M = max(nums)

    # Iterate over the range [1, M]
    for i in range(1, M + 1):
      
          # Stores the GCD of subsequence
        gcd = 0

        # Check if i can be the GCD of
        # any subsequence
        for j in range(i, M + 1, i):
            if j in s:
                gcd = math.gcd(gcd, j)

        # Store the value i in Ans[]
        # if it can be the GCD
        if gcd == i:
            Ans += [i]

    # Print all possible GCDs stored
    print(*Ans)


# Driver Code
N = 7
arr = [3, 4, 8]

# Function Call
findGCDsSubsequence(arr)

// C# program for the above approach
using System;
using System.Collections.Generic;

class GFG{

static int gcd1(int a, int b)
{
    return b == 0 ? a : gcd1(b, a % b);
}

// Function to find the different GCDs of
// the subsequence of the given array
static void findGCDsSubsequence(List<int> arr)
{
    
    // Stores all the possible GCDs
    List<int> ans = new List<int>();

    // Stores all array element in set
    HashSet<int> s = new HashSet<int>();
    foreach(int i in arr)
        s.Add(i);
        
    int M = Int32.MinValue;
    foreach(int i in arr)
    {
        if (i > M)
            M = i;
    }

    // Iterate over the range [1, M]
    for(int i = 1; i <= M; i++) 
    {
        int gcd = 0;

        // Check if i can be the GCD of
        // any subsequence
        for(int j = i; j < M + 1; j += i) 
        {
            if (s.Contains(j))
                gcd = gcd1(gcd, j);
        }
        if (gcd == i)
            ans.Add(i);
    }
    for(int i = 0; i < ans.Count; i++)
        Console.Write(ans[i] + " ");
}

// Driver Code
public static void Main()
{
    List<int> arr = new List<int>(){ 3, 4, 8 };

    // Function Call
    findGCDsSubsequence(arr);
}
}

// This code is contributed by SURENDRA_GANGWAR

<script>

// JavaScript program for the above approach

function __gcd(a, b) {
    if (b == 0)
    return a;
    return __gcd(b, a % b);
 }
 

// Function to find the different GCDs of
// the subsequence of the given array
function findGCDsSubsequence(arr) {

    // Stores all the possible GCDs
    let ans = [];

    // Stores all array element in set
    let s = new Set();
    for (let i of arr)
        s.add(i);

    let M = [...arr].sort((a, b) => b - a)[0]

    // Iterate over the range [1, M]
    for (let i = 1; i <= M; i++) {
        let gcd = 0;

        // Check if i can be the GCD of
        // any subsequence
        for (let j = i; j < M + 1; j += i) {
            if (s.has(j))
                gcd = __gcd(gcd, j);
        }
        if (gcd == i)
            ans.push(i);
    }
    for (let i = 0; i < ans.length; i++)
        document.write(ans[i] + " ");
}

// Driver Code

let N = 7;
let arr = [3, 4, 8];

// Function Call
findGCDsSubsequence(arr);

// This code is contributed by gfgking

</script>

1 3 4 8

Time Complexity: O(M*log M), where M is the maximum element of the array.
Auxiliary Space: O(N)