Count of elements such that difference between sum of left and right sub arrays is equal to a multiple of k

Given an array arr[] of length n and an integer k, the task is to find the number of indices from 2 to n-1 in the array having a difference of the sum of left and right sub array equal to the multiple of given number k.

Examples:

Input: arr[] = {1, 2, 3, 3, 1, 1}, k = 4
Output: 2
Explanation: The only possible indices are 4 and 5

Input: arr[] = {1, 2, 3, 4, 5}, k = 1
Output: 3

Approach:



  • Create a prefix array which contains the sum of the elements in the left and the suffix array which contains the sum of elements in the right.
  • Check for every index the difference of sum in the left and right and increase the counter if it is divisible by k

Below is the implementation of the above approach:

CPP

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// C++ code to count of elements such that 
// difference between the sum of left and right 
// sub-arrays are equal to a multiple of k
  
#include <bits/stdc++.h>
using namespace std;
  
// Functions to find the no of elements
int noOfElement(int a[], int n, int k)
{
    // Creating a prefix array
    int prefix[n];
  
    // Starting element of prefix array
    // will be the first element
    // of given array
    prefix[0] = a[0];
    for (int i = 1; i < n; i++) {
        prefix[i] = prefix[i - 1] + a[i];
    }
  
    // Creating a suffix array;
    int suffix[n];
    // Last element of suffix array will
    // be the last element of given array
    suffix[n - 1] = a[n - 1];
    for (int i = n - 2; i >= 0; i--) {
        suffix[i] = suffix[i + 1] + a[i];
    }
  
    // Checking difference of left and right half
    // is divisible by k or not.
    int cnt = 0;
    for (int i = 1; i < n - 1; i++) {
        if ((prefix[i] - suffix[i]) % k == 0
            || (suffix[i] - prefix[i]) % k == 0) {
            cnt = cnt + 1;
        }
    }
  
    return cnt;
}
  
// Driver code
int main()
{
    int a[] = { 1, 2, 3, 3, 1, 1 };
    int k = 4;
    int n = sizeof(a) / sizeof(a[0]);
    cout << noOfElement(a, n, k);
    return 0;
}

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Java

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// Java code to count of elements such that 
// difference between the sum of left and right 
// sub-arrays are equal to a multiple of k
class GFG
{
  
// Functions to find the no of elements
static int noOfElement(int a[], int n, int k)
{
    // Creating a prefix array
    int []prefix = new int[n];
  
    // Starting element of prefix array
    // will be the first element
    // of given array
    prefix[0] = a[0];
    for (int i = 1; i < n; i++)
    {
        prefix[i] = prefix[i - 1] + a[i];
    }
  
    // Creating a suffix array;
    int []suffix = new int[n];
      
    // Last element of suffix array will
    // be the last element of given array
    suffix[n - 1] = a[n - 1];
    for (int i = n - 2; i >= 0; i--)
    {
        suffix[i] = suffix[i + 1] + a[i];
    }
  
    // Checking difference of left and right half
    // is divisible by k or not.
    int cnt = 0;
    for (int i = 1; i < n - 1; i++) 
    {
        if ((prefix[i] - suffix[i]) % k == 0
            || (suffix[i] - prefix[i]) % k == 0
        {
            cnt = cnt + 1;
        }
    }
    return cnt;
}
  
// Driver code
public static void main(String[] args)
{
    int a[] = { 1, 2, 3, 3, 1, 1 };
    int k = 4;
    int n = a.length;
    System.out.print(noOfElement(a, n, k));
}
}
  
// This code is contributed by Rajput-Ji

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Python

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# Python3 code to count of elements such that
# difference between the sum of left and right
# sub-arrays are equal to a multiple of k
  
# Functions to find the no of elements
def noOfElement(a, n, k):
      
    # Creating a prefix array
    prefix = [0] * n
  
    # Starting element of prefix array
    # will be the first element
    # of given array
    prefix[0] = a[0]
    for i in range(1, n):
        prefix[i] = prefix[i - 1] + a[i]
  
    # Creating a suffix array
    suffix = [0] * n
      
    # Last element of suffix array will
    # be the last element of given array
    suffix[n - 1] = a[n - 1]
    for i in range(n - 2, -1, -1):
        suffix[i] = suffix[i + 1] + a[i]
  
    # Checking difference of left and right half
    # is divisible by k or not.
    cnt = 0
    for i in range(1, n - 1):
        if ((prefix[i] - suffix[i]) % k == 0 or (suffix[i] - prefix[i]) % k == 0):
            cnt = cnt + 1
  
    return cnt
  
# Driver code
  
a = [ 1, 2, 3, 3, 1, 1 ]
k = 4
n = len(a)
print(noOfElement(a, n, k))
  
# This code is contributed by mohit kumar 29

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C#

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// C# code to count of elements such that 
// difference between the sum of left and right 
// sub-arrays are equal to a multiple of k
using System;
  
class GFG
{
  
    // Functions to find the no of elements
    static int noOfElement(int []a, int n, int k)
    {
        // Creating a prefix array
        int []prefix = new int[n];
      
        // Starting element of prefix array
        // will be the first element
        // of given array
        prefix[0] = a[0];
        for (int i = 1; i < n; i++)
        {
            prefix[i] = prefix[i - 1] + a[i];
        }
      
        // Creating a suffix array;
        int []suffix = new int[n];
          
        // Last element of suffix array will
        // be the last element of given array
        suffix[n - 1] = a[n - 1];
        for (int i = n - 2; i >= 0; i--)
        {
            suffix[i] = suffix[i + 1] + a[i];
        }
      
        // Checking difference of left and right half
        // is divisible by k or not.
        int cnt = 0;
        for (int i = 1; i < n - 1; i++) 
        {
            if ((prefix[i] - suffix[i]) % k == 0
                || (suffix[i] - prefix[i]) % k == 0) 
            {
                cnt = cnt + 1;
            }
        }
        return cnt;
    }
      
    // Driver code
    public static void Main()
    {
        int []a = { 1, 2, 3, 3, 1, 1 };
        int k = 4;
        int n = a.Length;
        Console.Write(noOfElement(a, n, k));
    }
}
  
// This code is contributed by AnkitRai01

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Output:

2

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