Subarray with equal occurences!

• Time : $O(n)$
• Space : $O(n)$

int Solution::solve(vector<int> &A, int B, int C) {
    unordered_map<int, int> freq{{0,1}};
    int b = 0, c = 0, res = 0;
    for(auto a : A) {
        if(a == B) b += 1;
        if(a == C) c += 1;
        res += freq[b-c];
        freq[b-c] += 1;
    }
    return res;
}