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Subsequences of size three in an array whose sum is divisible by m
  • Difficulty Level : Medium
  • Last Updated : 27 Apr, 2021

Given an array A[] (1<=A_i  <=10^9  ) of size N (1<=N<=10^5  ), find the number of subsequences of length 3 whose sum is divisible by M (1<=M<=10^3  ). 

Examples: 

Input  : A[] = {1, 2, 4, 3}
           M = 3 
Output : 2
Explanation : We can choose two such 
subsequence of length 3 such that its 
sum is divisible by 3 -> {1, 2, 3} ans 
{2, 4, 3} 

Brute Force Approach: We use 3 nested loops to traverse all the subsequences of length 3 to count all such subsequence which satisfies the given condition.
Here the brute force solution will work in O(N^3
 

C++




// Brute Force Implementation (Time Complexity :
// O(N^3)) C++ program to find count of subsequences of size
// three divisible by M.
#include <bits/stdc++.h>
 
using namespace std;
 
int coutSubSeq(int A[], int N, int M)
{
    int sum = 0;
    int ans = 0;
 
    // Three nested loop to find all the sub
    // sequences of length three in the given
    // array A[].
    for (int i = 0; i < N; i++) {
        for (int j = i + 1; j < N; j++) {
            for (int k = j + 1; k < N; k++) {
                sum = A[i] + A[j] + A[k];
 
                // checking if the sum of the chosen
                // three number is divisible by m.
                if (sum % M == 0)
                    ans++;
            }
        }
    }
    return ans;
}
 
// Driver code
int main()
{
    int M = 3;
    int A[] = { 1, 2, 4, 3 };
    int N = sizeof(A)/sizeof(A[0]);
    cout << coutSubSeq(A, N, M);
    return 0;
}

Java




// Java program to find count of
// subsequences of size three
// divisible by M.
import java.io.*;
 
class GFG {
     
    static int coutSubSeq(int A[], int N,
                                   int M)
    {
        int sum = 0;
        int ans = 0;
      
        // Three nested loop to find all the
        // sub sequences of length three in
        // the given array A[].
        for (int i = 0; i < N; i++) {
            for (int j = i + 1; j < N; j++) {
                for (int k = j + 1; k < N; k++) {
                    sum = A[i] + A[j] + A[k];
      
                    // checking if the sum of the
                    // chosen three number is
                    // divisible by m.
                    if (sum % M == 0)
                        ans++;
                }
            }
        }
        return ans;
    }
      
    // Driver code
    public static void main(String args[])
    {
        int M = 3;
        int A[] = { 1, 2, 4, 3 };
        int N = A.length;
        System.out.println(coutSubSeq(A, N, M));
    }
}
 
/*This code is contributed by Nikita Tiwari*/

Python




# Python program to find count of
# subsequences of size three
# divisible by M.
 
def coutSubSeq(A, N,M) :
    sum = 0
    ans = 0
  
    # Three nested loop to find all the
    # sub sequences of length three in
    # the given array A[].
    for i in range(0, N) :
        for j in range(i + 1, N) :
            for k in range(j + 1, N) :
                sum = A[i] + A[j] + A[k]
  
                # checking if the sum of
                # the chosen three number
                # is divisible by m.
                if (sum % M == 0) :
                    ans = ans + 1
    return ans
     
# Driver code
M = 3
A = [ 1, 2, 4, 3 ]
N = len(A)
print coutSubSeq(A, N, M)
 
# This code is contributed by Nikita Tiwari.

C#




// C# program to find count of subsequences
// of size three divisible by M.
using System;
 
class GFG {
     
    static int coutSubSeq(int[] A, int N, int M)
    {
        int sum = 0;
        int ans = 0;
     
        // Three nested loop to find all the
        // sub sequences of length three in
        // the given array A[].
        for (int i = 0; i < N; i++) {
            for (int j = i + 1; j < N; j++) {
                for (int k = j + 1; k < N; k++)
                {
                    sum = A[i] + A[j] + A[k];
     
                    // checking if the sum of
                    // the chosen three number
                    // is divisible by m.
                    if (sum % M == 0)
                        ans++;
                }
            }
        }
        return ans;
    }
     
    // Driver code
    public static void Main()
    {
        int M = 3;
        int []A = { 1, 2, 4, 3 };
        int N = A.Length;
        Console.WriteLine(coutSubSeq(A, N, M));
    }
}
 
// This code is contributed by anuj_67.

PHP




<?php
// Brute Force Implementation (Time Complexity :
// O(N^3)) PHP program to find count of subsequences of size
// three divisible by M.
 
function coutSubSeq($A, $N, $M)
{
    $sum = 0;
    $ans = 0;
 
    // Three nested loop to find all the sub
    // sequences of length three in the given
    // array A[].
    for ( $i = 0; $i <$N; $i++) {
        for ( $j = $i + 1; $j < $N; $j++) {
            for ( $k = $j + 1; $k < $N; $k++) {
                $sum = $A[$i] + $A[$j] + $A[$k];
 
                // checking if the sum of the
                // chosen three number is
                // divisible by m.
                if ($sum % $M == 0)
                    $ans++;
            }
        }
    }
     
    return $ans;
}
 
// Driver code
    $M = 3;
    $A = array( 1, 2, 4, 3 );
    $N = count($A);
    echo coutSubSeq($A, $N, $M);
 
// This code is contributed by anuj_67.
?>

Javascript




<script>
 
// Javascript program to find count of
// subsequences of size three
// divisible by M.
 
    function coutSubSeq(A, N, M)
    {
        let sum = 0;
        let ans = 0;
        
        // Three nested loop to find all the
        // sub sequences of length three in
        // the given array A[].
        for (let i = 0; i < N; i++) {
            for (let j = i + 1; j < N; j++) {
                for (let k = j + 1; k < N; k++) {
                    sum = A[i] + A[j] + A[k];
        
                    // checking if the sum of the
                    // chosen three number is
                    // divisible by m.
                    if (sum % M == 0)
                        ans++;
                }
            }
        }
        return ans;
    }
     
// Driver code
 
        let M = 3;
        let A = [ 1, 2, 4, 3 ];
        let N = A.length;
        document.write(coutSubSeq(A, N, M));
                       
</script>

Output: 
 

2

Second Approach (Efficient for small M): The above implementation takes a lot of time. So we need a better approach to solve this. Here we will use index mapping (hashing technique) to solve the problem. As we need to check if the sum of the chosen three numbers is divisible by M, we store each value under modulo M. We use a frequency array to store the number of occurrences of each element using index mapping techniques.
Three cases may occur: 
Firstly, if thrice of a number is divisible by M then we will add {{N}\choose{3}}  to answer where N is frequency of that number. 
Secondly, if twice of some number A added with sum number B is divisible by M then we will add {{Freq[A]}\choose{2}}  * Freq[B] to the answer. 
Thirdly, if all the numbers A, B, C that sum up to S which is divisible by M then we will add Freq[A] * Freq[B] * Freq[C] to the answer.
With this approach, we can solve the problem in O(M^2  ) which is executable in the given time. 
 



C++




// O(M^2) time complexity C++ program to find
// count of subsequences of size three
// divisible by M.
#include <iostream>
 
using namespace std;
 
int countSubSeq(int A[], int N, int M)
{
    int ans = 0;
 
    // Storing frequencies of all remainders
    // when divided by M.
    int h[M] = { 0 };
    for (int i = 0; i < N; i++) {
        A[i] = A[i] % M;
        h[A[i]]++;
    }
 
    for (int i = 0; i < M; i++) {
        for (int j = i; j < M; j++) {
 
            // including i and j in the sum rem
            // calculate the remainder required
            // to make the sum divisible by M
            int rem = (M - (i + j) % M) % M;
 
            // if the required number is less than
            // j, we skip as we have already calculated
            // for that value before. As j here starts
            // with i and rem is less than j.
            if (rem < j)
                continue;
 
            // if satisfies the first case.
            if (i == j && rem == j)
                ans += h[i] * (h[i] - 1) * (h[i] - 2) / 6;
 
            // if satisfies the second case
            else if (i == j)
                ans += h[i] * (h[i] - 1) * h[rem] / 2;
 
            else if (i == rem)
                ans += h[i] * (h[i] - 1) * h[j] / 2;
            else if (rem == j)
                ans += h[j] * (h[j] - 1) * h[i] / 2;
 
            // if satisfies the third case
            else
                ans = ans + h[i] * h[j] * h[rem];
        }
    }
    return ans;
}
 
// Driver code
int main()
{  
    int M = 3;
    int A[] = { 1, 2, 4, 3 };
    int N = sizeof(A)/sizeof(A[0]);
    cout << countSubSeq(A, N, M);
    return 0;
}

Java




// Java program to find count of
// subsequences of size three
// divisible by M.
import java.io.*;
import java.util.Arrays;
 
class GFG {
 
    static int countSubSeq(int A[], int N, int M)
    {
        int ans = 0;
      
        // Storing frequencies of all
        // remainders when divided by M.
        int h[] = new int[M];
        Arrays.fill(h,0);
        for (int i = 0; i < N; i++) {
            A[i] = A[i] % M;
            h[A[i]]++;
        }
      
        for (int i = 0; i < M; i++) {
            for (int j = i; j < M; j++) {
      
                // including i and j in the sum
                // rem calculate the remainder
                // required to make the sum
                // divisible by M
                int rem = (M - (i + j) % M) % M;
      
                // if the required number is
                // less than j, we skip as we
                // have already calculated for
                // that value before. As j here
                // starts with i and rem is
                // less than j.
                if (rem < j)
                    continue;
      
                // if satisfies the first case.
                if (i == j && rem == j)
                    ans += h[i] * (h[i] - 1) *
                           (h[i] - 2) / 6;
      
                // if satisfies the second case
                else if (i == j)
                    ans += h[i] * (h[i] - 1) *
                           h[rem] / 2;
      
                else if (i == rem)
                    ans += h[i] * (h[i] - 1) *
                           h[j] / 2;
                else if (rem == j)
                    ans += h[j] * (h[j] - 1) *
                           h[i] / 2;
      
                // if satisfies the third case
                else
                    ans = ans + h[i] * h[j] *
                          h[rem];
            }
        }
        return ans;
    }
      
    // Driver code
    public static void main(String args[])
    {
        int M = 3;
        int A[] = { 1, 2, 4, 3 };
        int N = A.length;
        System.out.println(countSubSeq(A, N, M));
    }
}
 
/*This code is contributed by Nikita Tiwari*/

Python




# Python program to find count of
# subsequences of size three
# divisible by M.
 
def countSubSeq(A, N, M) :
    ans = 0
     
    # Storing frequencies of all
    # remainders when divided by M.
    h = [0] * M
    for i in range(0,N) :
        A[i] = A[i] % M
        h[A[i]] = h[A[i]] + 1
     
    for i in range(0,M) :
        for j in range(i,M) :
             
            # including i and j in the sum
            # rem calculate the remainder
            # required to make the sum
            # divisible by M
            rem = (M - (i + j) % M) % M
             
            # if the required number is
            # less than j, we skip as we
            # have already calculated for
            # that value before. As j here
            # starts with i and rem is
            # less than j.
            if (rem < j) :
                continue
                 
            # if satisfies the first case.
            if (i == j and rem == j) :
                ans = ans + h[i] * (h[i] - 1) * (h[i] - 2) / 6
                 
                 
            # if satisfies the second case
            elif (i == j) :
                ans = ans +( h[i] * (h[i] - 1) * h[rem] / 2)
      
            elif (i == rem) :
                ans = ans + h[i] * (h[i] - 1) * h[j] / 2
            elif (rem == j) :
                ans = ans + h[j] * (h[j] - 1) * h[i] / 2
      
            # if satisfies the third case
            else :
                ans = ans + h[i] * h[j] * h[rem]
             
        return ans
         
# Driver code
M = 3;
A = [ 1, 2, 4, 3 ]
N = len(A)
print(countSubSeq(A, N, M))
 
# This code is contributed by Nikita Tiwari.

C#




// C# program to find count of subsequences of
// size three divisible by M.
using System;
 
class GFG {
 
    static int countSubSeq(int []A, int N, int M)
    {
        int ans = 0;
     
        // Storing frequencies of all
        // remainders when divided by M.
        int []h = new int[M];
     
        for (int i = 0; i < N; i++) {
            A[i] = A[i] % M;
            h[A[i]]++;
        }
     
        for (int i = 0; i < M; i++) {
            for (int j = i; j < M; j++) {
     
                // including i and j in the sum
                // rem calculate the remainder
                // required to make the sum
                // divisible by M
                int rem = (M - (i + j) % M) % M;
     
                // if the required number is
                // less than j, we skip as we
                // have already calculated for
                // that value before. As j here
                // starts with i and rem is
                // less than j.
                if (rem < j)
                    continue;
     
                // if satisfies the first case.
                if (i == j && rem == j)
                    ans += h[i] * (h[i] - 1) *
                                (h[i] - 2) / 6;
     
                // if satisfies the second case
                else if (i == j)
                    ans += h[i] * (h[i] - 1) *
                                    h[rem] / 2;
     
                else if (i == rem)
                    ans += h[i] * (h[i] - 1) *
                                     h[j] / 2;
                else if (rem == j)
                    ans += h[j] * (h[j] - 1) *
                                     h[i] / 2;
     
                // if satisfies the third case
                else
                    ans = ans + h[i] * h[j] *
                                      h[rem];
            }
        }
        return ans;
    }
     
    // Driver code
    public static void Main()
    {
        int M = 3;
        int []A = { 1, 2, 4, 3 };
        int N = A.Length;
        Console.WriteLine(countSubSeq(A, N, M));
    }
}
 
// This code is contributed by anuj_67.

PHP




<?php
// O(M^2) time complexity PHP program to find
// count of subsequences of size three
// divisible by M.
 
function countSubSeq($A, $N, $M)
{
    $ans = 0;
 
    // Storing frequencies of all remainders
    // when divided by M.
    $h = array_fill(0, $M, 0);
    for ($i = 0; $i < $N; $i++)
    {
        $A[$i] = $A[$i] % $M;
        $h[$A[$i]]++;
    }
 
    for ($i = 0; $i < $M; $i++)
    {
        for ($j = $i; $j < $M; $j++)
        {
 
            // including i and j in the sum rem
            // calculate the remainder required
            // to make the sum divisible by M
            $rem = ($M - ($i + $j) % $M) % $M;
 
            // if the required number is less than
            // j, we skip as we have already calculated
            // for that value before. As j here starts
            // with i and rem is less than j.
            if ($rem < $j)
                continue;
 
            // if satisfies the first case.
            if ($i == $j && $rem == $j)
                $ans += $h[$i] * ($h[$i] - 1) *
                                 ($h[$i] - 2) / 6;
 
            // if satisfies the second case
            else if ($i == $j)
                $ans += $h[$i] * ($h[$i] - 1) *
                                  $h[$rem] / 2;
 
            else if ($i == $rem)
                $ans += $h[$i] * ($h[$i] - 1) *
                                  $h[$j] / 2;
            else if ($rem == $j)
                $ans += $h[$j] * ($h[$j] - 1) *
                                  $h[$i] / 2;
 
            // if satisfies the third case
            else
                $ans = $ans + $h[$i] *
                              $h[$j] * $h[$rem];
        }
    }
    return $ans;
}
 
// Driver code
$M = 3;
$A = array( 1, 2, 4, 3 );
$N = count($A);
echo countSubSeq($A, $N, $M);
     
// This code is contributed by mits
?>

Output: 
 

2

 

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