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# Count subarrays with equal number of occurrences of two given elements in C++

We are given an array arr[] of integers. Also, two numbers A and B. The goal is to count all subarrays of arr[] such that occurrences of A and B is equal in all. If the array is [1,2,3] and A is 1 and B is 2. Subarrays will be [3], [1,2], [1,2,3].

Let us understand with examples.

**Input** − arr[] = { 2, 2, 1, 1, 1, 5 }; A=1, B=5

**Output** − Count of subarrays with equal no. of occurrences of two given elements are − 4

**Explanation** − Subaarays will be − [2], [2], [2,2], [1,5]. First three have 0 occurrences of 1 and 5, last one has 1 occurrence of both.

**Input** − arr[] = { 5,3,7,5,3 }; A=1, B=2

**Output** − Count of subarrays with equal no. of occurrences of two given elements are − 15

**Explanation** − Subaarays will be − ( that have 0 occurrences of 1 and 2 )

[5], [3], [7], [5], [3] - 5 [5,3], [3,7], [7,5], [5,3] - 4 [5,3,7], [3,7,5], [7,5,3] - 3 [5,3,7,5], [3,7,5,3] - 2 [5,3,5,7,5] - 1

Total 15 subarrays with no occurrence ( 0 occurrences) of both 1 and 2.

## The approach used in the below program is as follows

We will traverse the array using two for loops to generate all subarrays possible. From i=0 to i<=size-1 and j=i to j<=size-1. Subarrays formed will be between arr[i] to arr[j]. Count frequency of A and B in each subarray. If equal then increment the count.

Take an array arr[] of numbers.

Function sub_EqualOccurrence(int arr[], int size, int A, int B) takes the array and returns a count of subarrays with equal no. of A and B.

Take the initial count as 0.

We will traverse the array using two for loops from i=0 to i<=size-1 and j=0 to j<=size-1.

Take two variables total_A, total_B as 0 for the number of A’s and B’s in subarray arr[i] to arr[j].

Compare arr[j] with A and B. If arr[j] is A or B then increment respective count ( total_A or total_B).

If total_A==total_B. Increment count. ( the subarray has the same number of A’s and B’s as elements).

At the end of both loops, return count as result.

## Example

#include <bits/stdc++.h> using namespace std; int sub_EqualOccurrence(int arr[], int size, int A, int B){ int count = 0; for (int i = 0; i <= size - 1; i++){ int total_A = 0; int total_B = 0; for (int j = i; j <= size - 1; j++){ if (arr[j] == A){ total_A++; } else if (arr[j] == B){ total_B++; } if(total_A == total_B){ count++; } } } return count; } // Driver code int main(){ int arr[] = { 2, 3, 1, 1, 4, 5 }; int size = sizeof(arr) / sizeof(arr[0]); int A = 1, B = 5; cout<<"Count of subarrays with equal number of occurrences of two given elements are: "<<sub_EqualOccurrence(arr, size, A, B); return (0); }

## Output

If we run the above code it will generate the following output −

Count of subarrays with equal number of occurrences of two given elements are: 5

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