2845. Count of Interesting Subarrays

You are given a 0-indexed integer array nums, an integer modulo, and an integer k.

Your task is to find the count of subarrays that are interesting.

A subarray nums[l..r] is interesting if the following condition holds:

• Let cnt be the number of indices i in the range [l, r] such that nums[i] % modulo == k. Then, cnt % modulo == k.

Return an integer denoting the count of interesting subarrays.

Note: A subarray is a contiguous non-empty sequence of elements within an array.

class Solution {
public:
    long long countInterestingSubarrays(vector<int>& nums, int modulo, int k) {
        int sum = 0;
        long long res = 0;
        unordered_map<int, int> freq{{0,1}};
        for(int i = 0; i < nums.size(); i++) {
            sum = sum + ((nums[i] % modulo) == k);
            sum %= modulo;
            res += freq[(sum - k + modulo) % modulo];
            freq[sum] += 1;
        }
        return res;
    }
};