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Generate an array with product of all subarrays of length exceeding one divisible by K
  • Last Updated : 14 Dec, 2020

Given two positive integers N and K, the task is to generate an array of length N such that the product of every subarray of length greater than 1 must be divisible by K and the maximum element of the array must be less than K. If no such array is possible, then print -1.

Examples:

Input: N = 3, K = 20
Output: {15, 12, 5}
Explanation: All subarrays of length greater than 1 are {15, 12}, {12, 5}, {15, 12, 5} and their corresponding products are 180, 60 and 900, which are all divisible by 20.

Input: N = 4, K = 100
Output: {90, 90, 90, 90}

Approach: The given problem can be solved by the following observations:



It can be observed that by making the product of every subarray of length 2 divisible by K, the product of every subarray of length greater than 2 will automatically be divisible by K.

Therefore, the idea is to take two divisors of K, say d1 and d2, such that d1 * d2 = K and place them alternatively in the array. Follow the steps below to solve the problem:

  1. Initialize two integer variables d1 and d2.
  2. Check if K is prime. If found to be true, print -1.
  3. Otherwise, calculate the divisors of K and store two divisors in d1 and d2.
  4. After that, traverse from i = 0 to N – 1.
  5. Print d1 if i is even. Otherwise, print d2.

Below is the implementation of the above approach:

C++

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// C++ program for the above approach
 
#include <iostream>
using namespace std;
 
// Function to check if the required
// array can be genrated or not
void array_divisbleby_k(int N, int K)
{
    // To check if divisor exists
    bool flag = false;
 
    // To store divisiors of K
    int d1, d2;
 
    // Check if K is prime or not
    for (int i = 2; i * i <= K; i++) {
 
        if (K % i == 0) {
            flag = true;
            d1 = i;
            d2 = K / i;
            break;
        }
    }
 
    // If array can be generated
    if (flag) {
 
        // Print d1 and d2 alternatively
        for (int i = 0; i < N; i++) {
 
            if (i % 2 == 1) {
                cout << d2 << " ";
            }
            else {
                cout << d1 << " ";
            }
        }
    }
 
    else {
 
        // No such array can be generated
        cout << -1;
    }
}
// Driver Code
int main()
{
    // Given N and K
    int N = 5, K = 21;
 
    // Function Call
    array_divisbleby_k(N, K);
 
    return 0;
}

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Java

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// Java program for the above approach
class GFG{
     
// Function to check if the required
// array can be genrated or not
public static void array_divisbleby_k(int N,
                                      int K)
{
     
    // To check if divisor exists
    boolean flag = false;
   
    // To store divisiors of K
    int d1 = 0, d2 = 0;
   
    // Check if K is prime or not
    for(int i = 2; i * i <= K; i++)
    {
        if (K % i == 0)
        {
            flag = true;
            d1 = i;
            d2 = K / i;
            break;
        }
    }
   
    // If array can be generated
    if (flag)
    {
         
        // Print d1 and d2 alternatively
        for(int i = 0; i < N; i++)
        {
            if (i % 2 == 1)
            {
                System.out.print(d2 + " ");
            }
            else
            {
                System.out.print(d1 + " ");
            }
        }
    }
   
    else
    {
         
        // No such array can be generated
        System.out.print(-1);
    }
}
 
// Driver Code
public static void main(String[] args)
{
     
    // Given N and K
    int N = 5, K = 21;
   
    // Function Call
    array_divisbleby_k(N, K);
}
}
 
// This code is contributed by divyesh072019

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Python3

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# Python3 program for the above approach
 
# Function to check if the required
# array can be genrated or not
def array_divisbleby_k(N, K):
 
    # To check if divisor exists
    flag = False
 
    # To store divisiors of K
    d1, d2 = 0, 0
 
    # Check if K is prime or not
    for i in range(2, int(K ** (1 / 2)) + 1):
        if (K % i == 0):
            flag = True
            d1 = i
            d2 = K // i
            break
 
    # If array can be generated
    if (flag):
 
        # Print d1 and d2 alternatively
        for i in range(N):
            if (i % 2 == 1):
                print(d2, end = " ")
            else:
                print(d1, end = " ")
                 
    else:
 
        # No such array can be generated
        print(-1)
  
# Driver Code
if __name__ == "__main__":
 
    # Given N and K
    N = 5
    K = 21
 
    # Function Call
    array_divisbleby_k(N, K)
 
# This code is contributed by AnkThon

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C#

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// C# program for the above approach
using System;
 
class GFG{
     
// Function to check if the required
// array can be genrated or not
public static void array_divisbleby_k(int N,
                                      int K)
{
     
    // To check if divisor exists
    bool flag = false;
     
    // To store divisiors of K
    int d1 = 0, d2 = 0;
   
    // Check if K is prime or not
    for(int i = 2; i * i <= K; i++)
    {
        if (K % i == 0)
        {
            flag = true;
            d1 = i;
            d2 = K / i;
            break;
        }
    }
   
    // If array can be generated
    if (flag)
    {
         
        // Print d1 and d2 alternatively
        for(int i = 0; i < N; i++)
        {
            if (i % 2 == 1)
            {
                Console.Write(d2 + " ");
            }
            else
            {
                Console.Write(d1 + " ");
            }
        }
    }
   
    else
    {
         
        // No such array can be generated
        Console.Write(-1);
    }
}
 
// Driver Code
public static void Main(string[] args)
{
     
    // Given N and K
    int N = 5, K = 21;
   
    // Function Call
    array_divisbleby_k(N, K);
}
}
 
// This code is contributed by AnkThon

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Output:

3 7 3 7 3

Time complexity: O(N + √K)
Auxiliary space: O(1)

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