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Minimum index to split array into subarrays with co-prime products
  • Last Updated : 17 Nov, 2020

Given an array arr[] consisting of N integers, the task is to find the maximum index K such that the product of subarrays {arr[0], arr[K]} and {arr[K + 1], arr[N – 1]} are co-prime. If no such index exists, then print “-1”.

Examples:

Input: arr[] = {2, 3, 4, 5}
Output: 2
Explanation:
Smallest index for partition is 2.
Product of left subarray is = 2 * 3 * 4 = 24.
Product of right subarray = 5.
Since 24 and 5 are co-prime, the required answer is 2.

Input: arr[] = {23, 41, 52, 83, 7, 13}
Output: 0
Explanation:
Smallest index for partition is 0.
Product of left subarray = 23.
Product of right subarray = 41 * 52 * 83 * 7 * 13 = 16102996.
Since 23 and 16102996 are co-prime, the answer is 0.

Naive Approach: The simplest approach is to check all possible index of partition from the start of the array and check if the product of the subarrays formed is co-prime or not. If there exists any such index then print that index. Otherwise, print “-1”
Time Complexity: O(N2)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above approach, the idea is to use the prefix product array and suffix product array and find the possible index. Follow the steps below to solve the problem:



  • Create two auxiliary arrays, prefix[] and suffix[] to store the prefix and suffix array product. Initialize prefix[0] to arr[0] and suffix[N – 1] to arr[N – 1].
  • Traverse the given array over the range [2, N] using variable i and update the prefix array as prefix[i] = prefix[i – 1]*arr[i].
  • Traverse the given array from the back over the range [N – 2, 0] using variable i and update the suffix array as suffix[i] = suffix[i + 1]*arr[i].
  • Iterate a loop over the range [0, N – 1] using variable i and check if prefix[i] and suffix[i + 1] are co-prime or not. If found to be true the print the current index and break out of the loop.
  • If there doesn’t exist any such index in the above step then print “-1”.

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 GCD of 2 numbers
int GCD(int a, int b)
{
    // Base Case
    if (b == 0)
        return a;
 
    // Find the GCD recursively
    return GCD(b, a % b);
}
 
// Function to find the minimum partition
// index K s.t. product of both subarrays
// around that partition are co-prime
int findPartition(int nums[], int N)
{
 
    // Stores the prefix and suffix
    // array product
    int prefix[N], suffix[N], i, k;
 
    prefix[0] = nums[0];
 
    // Update the prefix array
    for (i = 1; i < N; i++) {
        prefix[i] = prefix[i - 1]
                    * nums[i];
    }
 
    suffix[N - 1] = nums[N - 1];
 
    // Update the suffix array
    for (i = N - 2; i >= 0; i--) {
        suffix[i] = suffix[i + 1]
                    * nums[i];
    }
 
    // Iterate the given array
    for (k = 0; k < N - 1; k++) {
        // Check if prefix[k] and
        // suffix[k+1] are co-prime
        if (GCD(prefix[k],
                suffix[k + 1])
            == 1) {
            return k;
        }
    }
 
    // If no index for partition
    // exists, then return -1
    return -1;
}
 
// Driver Code
int main()
{
 
    int arr[] = { 2, 3, 4, 5 };
 
    int N = sizeof(arr) / sizeof(arr[0]);
 
    // Function call
    cout << findPartition(arr, N);
 
    return 0;
}

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Java

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// Java program for the
// above approach
import java.util.*;
class solution{
    
// Function to find the
// GCD of 2 numbers
static int GCD(int a,
               int b)
{
  // Base Case
  if (b == 0)
    return a;
 
  // Find the GCD
  // recursively
  return GCD(b, a % b);
}
 
// Function to find the minimum
// partition index K s.t. product
// of both subarrays around that
// partition are co-prime
static int findPartition(int nums[],
                         int N)
{
  // Stores the prefix and
  // suffix array product
  int []prefix = new int[N];
  int []suffix = new int[N];
  int i, k;
 
  prefix[0] = nums[0];
 
  // Update the prefix array
  for (i = 1; i < N; i++)
  {
    prefix[i] = prefix[i - 1] *
                nums[i];
  }
 
  suffix[N - 1] = nums[N - 1];
 
  // Update the suffix array
  for (i = N - 2; i >= 0; i--)
  {
    suffix[i] = suffix[i + 1] *
                nums[i];
  }
 
  // Iterate the given array
  for (k = 0; k < N - 1; k++)
  {
    // Check if prefix[k] and
    // suffix[k+1] are co-prime
    if (GCD(prefix[k],
            suffix[k + 1]) == 1)
    {
      return k;
    }
  }
 
  // If no index for partition
  // exists, then return -1
  return -1;
}
 
// Driver Code
public static void main(String args[])
{
  int arr[] = {2, 3, 4, 5};
  int N = arr.length;
 
  // Function call
  System.out.println(findPartition(arr, N));
}
}
 
// This code is contributed by SURENDRA_GANGWAR

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Python3

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# Python3 program for the
# above approach
 
# Function to find the
# GCD of 2 numbers
def GCD(a, b):
   
    # Base Case
    if (b == 0):
        return a
 
    # Find the GCD recursively
    return GCD(b, a % b)
 
# Function to find the minimum
# partition index K s.t. product
# of both subarrays around that
# partition are co-prime
def findPartition(nums, N):
 
    #Stores the prefix and
    # suffix array product
    prefix=[0] * N
    suffix=[0] * N
 
    prefix[0] = nums[0]
 
    # Update the prefix
    # array
    for i in range(1, N):
        prefix[i] = (prefix[i - 1] *
                     nums[i])
 
    suffix[N - 1] = nums[N - 1]
 
    # Update the suffix array
    for i in range(N - 2, -1, -1):
        suffix[i] = (suffix[i + 1] *
                     nums[i])
 
    # Iterate the given array
    for k in range(N - 1):
       
        # Check if prefix[k] and
        # suffix[k+1] are co-prime
        if (GCD(prefix[k],
                suffix[k + 1]) == 1):
            return k
 
    # If no index for partition
    # exists, then return -1
    return -1
 
# Driver Code
if __name__ == '__main__':
   
    arr = [2, 3, 4, 5]
    N = len(arr)
 
    # Function call
    print(findPartition(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
// GCD of 2 numbers
static int GCD(int a, int b)
{
  // Base Case
  if (b == 0)
    return a;
 
  // Find the GCD
  // recursively
  return GCD(b, a % b);
}
 
// Function to find the minimum
// partition index K s.t. product
// of both subarrays around that
// partition are co-prime
static int findPartition(int[] nums,
                         int N)
{
  // Stores the prefix and
  // suffix array product
  int[] prefix = new int[N];
  int[] suffix = new int[N];
  int i, k;
 
  prefix[0] = nums[0];
 
  // Update the prefix array
  for (i = 1; i < N; i++)
  {
    prefix[i] = prefix[i - 1] *
                nums[i];
  }
 
  suffix[N - 1] = nums[N - 1];
 
  // Update the suffix array
  for (i = N - 2; i >= 0; i--)
  {
    suffix[i] = suffix[i + 1] *
                nums[i];
  }
 
  // Iterate the given array
  for (k = 0; k < N - 1; k++)
  {
    // Check if prefix[k] and
    // suffix[k+1] are co-prime
    if (GCD(prefix[k],
            suffix[k + 1]) == 1)
    {
      return k;
    }
  }
 
  // If no index for partition
  // exists, then return -1
  return -1;
}
 
// Driver code
static void Main()
{
  int[] arr = {2, 3, 4, 5};
  int N = arr.Length;
 
  // Function call
  Console.WriteLine(findPartition(arr, N));
}
}
 
// This code is contributed by divyeshrabadiya07

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Output: 

2








 

Time Complexity: O(N log(N))
Auxiliary Space: O(N)

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