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Remove an element to maximize the GCD of the given array

  • Difficulty Level : Hard
  • Last Updated : 07 May, 2021

Given an array arr[] of length N ≥ 2. The task is to remove an element from the given array such that the GCD of the array after removing it is maximized.

Examples: 

Input: arr[] = {12, 15, 18} 
Output: 6 
Remove 12: GCD(15, 18) = 3 
Remove 15: GCD(12, 18) = 6 
Remove 18: GCD(12, 15) = 3

Input: arr[] = {14, 17, 28, 70} 
Output: 14 

Approach:  

  • Idea is to find the GCD value of all the sub-sequences of length (N – 1) and removing the element which is not present in the sub-sequence with that GCD. The maximum GCD found would be the answer.
  • To find the GCD of the sub-sequences optimally, maintain a prefixGCD[] and a suffixGCD[] array using single state dynamic programming.
  • The maximum value of GCD(prefixGCD[i – 1], suffixGCD[i + 1]) is the required answer.

Below is the implementation of the above approach:  

C++




// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to return the maximized gcd
// after removing a single element
// from the given array
int MaxGCD(int a[], int n)
{
 
    // Prefix and Suffix arrays
    int Prefix[n + 2];
    int Suffix[n + 2];
 
    // Single state dynamic programming relation
    // for storing gcd of first i elements
    // from the left in Prefix[i]
    Prefix[1] = a[0];
    for (int i = 2; i <= n; i += 1) {
        Prefix[i] = __gcd(Prefix[i - 1], a[i - 1]);
    }
 
    // Initializing Suffix array
    Suffix[n] = a[n - 1];
 
    // Single state dynamic programming relation
    // for storing gcd of all the elements having
    // greater than or equal to i in Suffix[i]
    for (int i = n - 1; i >= 1; i -= 1) {
        Suffix[i] = __gcd(Suffix[i + 1], a[i - 1]);
    }
 
    // If first or last element of
    // the array has to be removed
    int ans = max(Suffix[2], Prefix[n - 1]);
 
    // If any other element is replaced
    for (int i = 2; i < n; i += 1) {
        ans = max(ans, __gcd(Prefix[i - 1], Suffix[i + 1]));
    }
 
    // Return the maximized gcd
    return ans;
}
 
// Driver code
int main()
{
    int a[] = { 14, 17, 28, 70 };
    int n = sizeof(a) / sizeof(a[0]);
 
    cout << MaxGCD(a, n);
 
    return 0;
}
Java



// Java implementation of the above approach
class Test
{
    // Recursive function to return gcd of a and b
    static int gcd(int a, int b)
    {
        if (b == 0)
            return a;
        return gcd(b, a % b);
    }
     
    // Function to return the maximized gcd
    // after removing a single element
    // from the given array
    static int MaxGCD(int a[], int n)
    {
     
        // Prefix and Suffix arrays
        int Prefix[] = new int[n + 2];
        int Suffix[] = new int[n + 2] ;
     
        // Single state dynamic programming relation
        // for storing gcd of first i elements
        // from the left in Prefix[i]
        Prefix[1] = a[0];
        for (int i = 2; i <= n; i += 1)
        {
            Prefix[i] = gcd(Prefix[i - 1], a[i - 1]);
        }
     
        // Initializing Suffix array
        Suffix[n] = a[n - 1];
     
        // Single state dynamic programming relation
        // for storing gcd of all the elements having
        // greater than or equal to i in Suffix[i]
        for (int i = n - 1; i >= 1; i -= 1)
        {
            Suffix[i] = gcd(Suffix[i + 1], a[i - 1]);
        }
     
        // If first or last element of
        // the array has to be removed
        int ans = Math.max(Suffix[2], Prefix[n - 1]);
     
        // If any other element is replaced
        for (int i = 2; i < n; i += 1)
        {
            ans = Math.max(ans, gcd(Prefix[i - 1], Suffix[i + 1]));
        }
     
        // Return the maximized gcd
        return ans;
    }
         
    // Driver code
    public static void main(String[] args)
    {
 
        int a[] = { 14, 17, 28, 70 };
        int n = a.length;
     
        System.out.println(MaxGCD(a, n));
    }
}
 
// This code is contributed by AnkitRai01
Python3



# Python3 implementation of the above approach
import math as mt
 
# Function to return the maximized gcd
# after removing a single element
# from the given array
 
def MaxGCD(a, n):
 
 
    # Prefix and Suffix arrays
    Prefix=[0 for i in range(n + 2)]
    Suffix=[0 for i in range(n + 2)]
 
    # Single state dynamic programming relation
    # for storing gcd of first i elements
    # from the left in Prefix[i]
    Prefix[1] = a[0]
    for i in range(2,n+1):
        Prefix[i] = mt.gcd(Prefix[i - 1], a[i - 1])
 
    # Initializing Suffix array
    Suffix[n] = a[n - 1]
 
    # Single state dynamic programming relation
    # for storing gcd of all the elements having
    # greater than or equal to i in Suffix[i]
    for i in range(n-1,0,-1):
        Suffix[i] =mt.gcd(Suffix[i + 1], a[i - 1])
 
    # If first or last element of
    # the array has to be removed
    ans = max(Suffix[2], Prefix[n - 1])
 
    # If any other element is replaced
    for i in range(2,n):
        ans = max(ans, mt.gcd(Prefix[i - 1], Suffix[i + 1]))
 
    # Return the maximized gcd
    return ans
 
# Driver code
 
a=[14, 17, 28, 70]
n = len(a)
 
print(MaxGCD(a, n))
 
# This code is contributed by mohit kumar 29
C#



// C# implementation of the above approach
using System;
 
class GFG
{
     
    // Recursive function to return gcd of a and b
    static int gcd(int a, int b)
    {
        if (b == 0)
            return a;
        return gcd(b, a % b);
    }
     
    // Function to return the maximized gcd
    // after removing a single element
    // from the given array
    static int MaxGCD(int []a, int n)
    {
     
        // Prefix and Suffix arrays
        int []Prefix = new int[n + 2];
        int []Suffix = new int[n + 2] ;
     
        // Single state dynamic programming relation
        // for storing gcd of first i elements
        // from the left in Prefix[i]
        Prefix[1] = a[0];
        for (int i = 2; i <= n; i += 1)
        {
            Prefix[i] = gcd(Prefix[i - 1], a[i - 1]);
        }
     
        // Initializing Suffix array
        Suffix[n] = a[n - 1];
     
        // Single state dynamic programming relation
        // for storing gcd of all the elements having
        // greater than or equal to i in Suffix[i]
        for (int i = n - 1; i >= 1; i -= 1)
        {
            Suffix[i] = gcd(Suffix[i + 1], a[i - 1]);
        }
     
        // If first or last element of
        // the array has to be removed
        int ans = Math.Max(Suffix[2], Prefix[n - 1]);
     
        // If any other element is replaced
        for (int i = 2; i < n; i += 1)
        {
            ans = Math.Max(ans, gcd(Prefix[i - 1], Suffix[i + 1]));
        }
     
        // Return the maximized gcd
        return ans;
    }
         
    // Driver code
    static public void Main ()
    {
         
        int []a = { 14, 17, 28, 70 };
        int n = a.Length;
     
        Console.Write(MaxGCD(a, n));
    }
}
 
// This code is contributed by ajit.
Javascript



<script>
 
// Javascript implementation of the above approach
 
// Recursive function to return gcd of a and b 
function gcd(a, b)
{
    if (b == 0)
        return a;
         
    return gcd(b, a % b);
}
     
// Function to return the maximized gcd
// after removing a single element
// from the given array
function MaxGCD(a, n)
{
     
    // Prefix and Suffix arrays
    let Prefix = new Array(n + 2);
    let Suffix = new Array(n + 2);
 
    // Single state dynamic programming relation
    // for storing gcd of first i elements
    // from the left in Prefix[i]
    Prefix[1] = a[0];
    for(let i = 2; i <= n; i += 1)
    {
        Prefix[i] = gcd(Prefix[i - 1], a[i - 1]);
    }
 
    // Initializing Suffix array
    Suffix[n] = a[n - 1];
 
    // Single state dynamic programming relation
    // for storing gcd of all the elements having
    // greater than or equal to i in Suffix[i]
    for(let i = n - 1; i >= 1; i -= 1)
    {
        Suffix[i] = gcd(Suffix[i + 1], a[i - 1]);
    }
 
    // If first or last element of
    // the array has to be removed
    let ans = Math.max(Suffix[2], Prefix[n - 1]);
 
    // If any other element is replaced
    for(let i = 2; i < n; i += 1)
    {
        ans = Math.max(ans, gcd(Prefix[i - 1],
                                Suffix[i + 1]));
    }
     
    // Return the maximized gcd
    return ans;
}
 
// Driver code
let a = [ 14, 17, 28, 70 ];
let n = a.length;
 
document.write(MaxGCD(a, n));
 
// This code is contributed by rishavmahato348
 
</script>
Output: 
14
 
Time Complexity: O(N * log(M)) where M is the maximum element from the array.
 

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