Subarrays with sum K

Given an unsorted array of integers, find the number of continuous subarrays having sum exactly equal to a given number k.

Examples:

Input: arr = [10, 2, -2, -20, 10], k = -10
Output: 3
Explaination: Subarrays: arr[0...3], arr[1...4], arr[3...4] have sum exactly equal to -10.

Input: arr = [9, 4, 20, 3, 10, 5], k = 33
Output: 2
Explaination: Subarrays: arr[0...2], arr[2...4] have sum exactly equal to 33.

Input: arr = [1, 3, 5], k = 0
Output: 0
Explaination: No subarray with 0 sum.

Constraints:

• 1 ≤ arr.size() ≤ 10⁵
• -10³ ≤ arr[i] ≤ 10³
• -10⁷ ≤ k ≤ 10⁷

class Solution {
  public:
    int countSubarrays(vector<int> &arr, int k) {

    // unordered_map to store prefix sums frequencies
    unordered_map<int, int> prefixSums;
  
    int res = 0;
    int currSum = 0;

    for (int i = 0; i < arr.size(); i++) {
        
        // Add current element to sum so far.
        currSum += arr[i];

        // If currSum is equal to desired sum, then a new
        // subarray is found. So increase count of subarrays.
        if (currSum == k)
            res++;

        // Check if the difference exists in the prefixSums map.
        if (prefixSums.find(currSum - k) != prefixSums.end())
            res += prefixSums[currSum - k];

        // Add currSum to the set of prefix sums.
        prefixSums[currSum]++;
    }

    return res;

    }
};