2489. Number of Substrings With Fixed Ratio

You are given a binary string s, and two integers num1 and num2. num1 and num2 are coprime numbers.

A ratio substring is a substring of s where the ratio between the number of 0’s and the number of 1’s in the substring is exactly num1 : num2.

• For example, if num1 = 2 and num2 = 3, then “01011” and “1110000111” are ratio substrings, while “11000” is not.

Return the number of non-empty ratio substrings of s.

Note that:

• A substring is a contiguous sequence of characters within a string.
• Two values x and y are coprime if gcd(x, y) == 1 where gcd(x, y) is the greatest common divisor of x and y.

class Solution {
public:
    long long fixedRatio(string s, int num1, int num2) {
        unordered_map<long long, int> freq{{0, 1}};
        long long res = 0, now = 0;
        for (auto& ch : s) {
            if (ch == '0') now += num2;
            else now -= num1;
            res += freq[now];
            freq[now] += 1;
        }
        return res;
    }
};