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Rearrange array to obtain maximum possible value of concatenation of prefix GCDs
  • Last Updated : 07 Oct, 2020

Given an array arr[] consisting of N positive integers, the task is to rearrange the array elements such that the number formed by concatenating the GCD of elements of the array arr[] from index 0 to i for each index i is the maximum possible.

Examples:

Input: arr[] = {4, 2, 5}
Output: 5 4 2
Explanation:
X = 511 is the maximum value of X that can be obtained among all the rearrangement of arr[].
Possible arrangements of arr[] are:
arr[] = [2, 4, 5] → X = 221
arr[] = [2, 5, 4] → X = 211
arr[] = [4, 2, 5] → X = 421
arr[] = [4, 5, 2] → X = 411
arr[] = [5, 4, 2] → X = 511
arr[] = [5, 2, 4] → X = 511

Input: arr[] = {2, 4, 6, 8}
Output: 8 4 6
Explanation: 
X = 842 is the maximum value of X that can be obtained among all the rearrangement of arr[].
Possible arrangements of arr[] are:
arr[] = [4, 6, 8] → X = 422
arr[] = [4, 8, 6] → X = 442
arr[] = [6, 4, 8] → X = 622
arr[] = [6, 8, 4] → X = 622
arr[] = [8, 4, 6] → X = 842
arr[] = [8, 6, 4] → X = 822

Approach: The GCD of a number alone is the number itself, thus the first digit of X i.e., X[0] would always be equal to arr[0]. Thus, to ensure that X is maximum among all obtainable numbers, arr[0] needs to be maximum. Then proceed by keeping track of the GCD of the longest prefix of arr[] that has been already arranged and find the values of the consecutive elements to be placed after this prefix. Follow the steps below to solve the above problem:



  1. The largest element of the array is set as the first element, thus the first prefix correctly arranged in the array arr[].
  2. Now find the element consecutive to the last element of the prefix i.e., arr[1].
  3. Here the GCD of the longest prefix(say G) is equal to arr[0], thus traverse the remaining array to find the element that gives the greatest GCD with G.
  4. Now, swap the element arr[1] with the element that gives maximum GCD with value G, update the value of G to this maximum GCD obtained i.e., G = GCD(G, arr[1]).
  5. Now the longest fixed prefix becomes arr[0], arr[1], continue this process for finding arr[2], arr[3], …, arr[N – 1], to obtain the required array.
  6. Print rearrange array after the above steps.

Below is the implementation of the above approach:

C++

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// C++ program for the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the maximum number
// obtainable from prefix GCDs
void prefixGCD(int arr[], int N)
{
    // Stores the GCD of the
    // longest prefix
    int gcc;
 
    // Sort the array
    sort(arr, arr + N);
 
    // Reverse the array
    reverse(arr, arr + N);
 
    // GCD of a[0] is a[0]
    gcc = arr[0];
    int start = 0;
 
    // Iterate to place the arr[start + 1]
    // element at it's correct position
    while (start < N - 1) {
 
        int g = 0, s1;
 
        for (int i = start + 1; i < N; i++) {
 
            // Find the element with
            // maximum GCD
            int gc = __gcd(gcc, arr[i]);
 
            // Update the value of g
            if (gc > g) {
                g = gc;
                s1 = i;
            }
        }
 
        // Update GCD of prefix
        gcc = g;
 
        // Place arr[s1] to it's
        // correct position
        swap(arr[s1], arr[start + 1]);
 
        // Increment start for the
        // remaining elements
        start++;
    }
 
    // Print the rearranged array
    for (int i = 0; i < N; i++) {
        cout << arr[i] << " ";
    }
}
 
// Driver Code
int main()
{
    // Given array arr[]
    int arr[] = { 1, 2, 3, 4 };
 
    int N = sizeof(arr) / sizeof(arr[0]);
 
    // Function Call
    prefixGCD(arr, N);
 
    return 0;
}

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Java

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//Java program for
// the above approach
import java.util.*;
class GFG{
 
//Function to find the maximum number
//obtainable from prefix GCDs
static void prefixGCD(int arr[], int N)
{
  // Stores the GCD of the
  // longest prefix
  int gcc;
 
  // Sort the array
  Arrays.sort(arr);
 
  // Reverse the array
  arr = reverse(arr);
 
  // GCD of a[0] is a[0]
  gcc = arr[0];
  int start = 0;
 
  // Iterate to place
  // the arr[start + 1]
  // element at it's
  // correct position
  while (start < N - 1)
  {
    int g = 0, s1 = 0;
 
    for (int i = start + 1; i < N; i++)
    {
      // Find the element with
      // maximum GCD
      int gc = __gcd(gcc, arr[i]);
 
      // Update the value of g
      if (gc > g)
      {
        g = gc;
        s1 = i;
      }
    }
 
    // Update GCD of prefix
    gcc = g;
 
    // Place arr[s1] to it's
    // correct position
    arr = swap(arr, s1, start + 1);
 
    // Increment start for the
    // remaining elements
    start++;
  }
 
  // Print the rearranged array
  for (int i = 0; i < N; i++)
  {
    System.out.print(arr[i] + " ");
  }
}
   
static int __gcd(int a, int b) 
  return b == 0 ? a : __gcd(b, a % b);    
}
 
static int[] reverse(int a[])
{
  int i, n = a.length, t;
  for (i = 0; i < n / 2; i++)
  {
    t = a[i];
    a[i] = a[n - i - 1];
    a[n - i - 1] = t;
  }
  return a;
}
 
static int[] swap(int []arr,
                  int i, int j)
{
  int temp = arr[i];
  arr[i] = arr[j];
  arr[j] = temp;
  return arr;
}
     
//Driver Code
public static void main(String[] args)
{
  // Given array arr[]
  int arr[] = {1, 2, 3, 4};
 
  int N = arr.length;
 
  // Function Call
  prefixGCD(arr, N);
}
}
 
//This code is contributed by 29AjayKumar

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Python3

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# Python3 program for the above approach
from math import gcd
 
# Function to find the maximum number
# obtainable from prefix GCDs
def prefixGCD(arr, N):
     
    # Stores the GCD of the
    # longest prefix
    gcc = 0
 
    # Sort the array
    arr = sorted(arr)
 
    # Reverse the array
    arr = arr[::-1]
 
    # GCD of a[0] is a[0]
    gcc = arr[0]
    start = 0
 
    # Iterate to place the arr[start + 1]
    # element at it's correct position
    while (start < N - 1):
        g = 0
        s1 = 0
 
        for i in range(start + 1, N):
 
            # Find the element with
            # maximum GCD
            gc = gcd(gcc, arr[i])
 
            # Update the value of g
            if (gc > g):
                g = gc
                s1 = i
 
        # Update GCD of prefix
        gcc = g
 
        # Place arr[s1] to it's
        # correct position
        arr[s1], arr[start + 1] = arr[start + 1], arr[s1]
 
        # Increment start for the
        # remaining elements
        start += 1
 
    # Print the rearranged array
    for i in range(N):
        print(arr[i], end = " ")
 
# Driver Code
if __name__ == '__main__':
     
    # Given array arr[]
    arr = [ 1, 2, 3, 4 ]
 
    N = len(arr)
 
    # Function Call
    prefixGCD(arr, N)
 
# This code is contributed by mohit kumar 29

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C#

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// C# program for the above approach 
using System;
class GFG{
 
// Function to find the maximum number
// obtainable from prefix GCDs
static void prefixGCD(int[] arr, int N)
{
     
  // Stores the GCD of the
  // longest prefix
  int gcc;
 
  // Sort the array
  Array.Sort(arr);
 
  // Reverse the array
  arr = reverse(arr);
 
  // GCD of a[0] is a[0]
  gcc = arr[0];
  int start = 0;
 
  // Iterate to place the
  // arr[start + 1] element
  // at it's correct position
  while (start < N - 1)
  {
    int g = 0, s1 = 0;
 
    for(int i = start + 1; i < N; i++)
    {
         
      // Find the element with
      // maximum GCD
      int gc = __gcd(gcc, arr[i]);
 
      // Update the value of g
      if (gc > g)
      {
        g = gc;
        s1 = i;
      }
    }
 
    // Update GCD of prefix
    gcc = g;
 
    // Place arr[s1] to it's
    // correct position
    arr = swap(arr, s1, start + 1);
 
    // Increment start for the
    // remaining elements
    start++;
  }
 
  // Print the rearranged array
  for(int i = 0; i < N; i++)
  {
    Console.Write(arr[i] + " ");
  }
}
   
static int __gcd(int a, int b) 
  return b == 0 ? a : __gcd(b, a % b);    
}
 
static int[] reverse(int[] a)
{
  int i, n = a.Length, t;
   
  for(i = 0; i < n / 2; i++)
  {
    t = a[i];
    a[i] = a[n - i - 1];
    a[n - i - 1] = t;
  }
  return a;
}
 
static int[] swap(int []arr, int i,
                             int j)
{
  int temp = arr[i];
  arr[i] = arr[j];
  arr[j] = temp;
  return arr;
}
     
//Driver Code
public static void Main()
{
     
  // Given array arr[]
  int[] arr = { 1, 2, 3, 4 };
 
  int N = arr.Length;
 
  // Function call
  prefixGCD(arr, N);
}
}
 
// This code is contributed by sanjoy_62

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Output: 

4 2 3 1






 

Time Complexity: O(N2)
Auxiliary Space: O(1)

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