LeetCode 2272. Substring With Largest Variance

class Solution {
public:
    int largestVariance(string s) {
        int ans = 0;
        unordered_set<char> unique(s.begin(), s.end());
        // go through each possibilities to find the ans.
        for (char a : unique)
            for (char b : unique) {
                if (a == b) continue;
                int var = 0, has_b = 0, first_b = 0;
                for (auto ch : s) {
                    // var is used to track the occurrences diff between "a" and "b".
                    if (ch == a) ++var;
                    else if (ch == b) {
                        has_b = true;
                        // we are having a leading "b" and now one more "b" with cnt(a) >= cnt(b),
                        // so we can cut the leading "b" for a better ans.
                        if (first_b && var >= 0) first_b = false;
                        // "--var < 0" indicates that we are having too many "b(s)" here,
                        // so we take this "b" as a fresh start.
                        else if (--var < 0) first_b = true, var = -1;
                    }
                    // update the ans if and only if "b" is within the substr.
                    ans = max(ans, has_b ? var : 0);
                }
            }
        return ans;
    }
};