Substring With Largest Variance — C++

// Time:  O(a^2 * n), a is the size of alphabets
// Space: O(a)

// kadane's algorithm
class Solution {
public:
    int largestVariance(string s) {
        const auto& modified_kadane = [&s](const auto& x, const auto& y) {
            int result = 0;
            vector<int> lookup(2);
            vector<int> remain = {static_cast<int>(count(cbegin(s), cend(s), x)),
                                  static_cast<int>(count(cbegin(s), cend(s), y))};
            int curr = 0;
            for (const auto& c : s) {
                if (!(c == x || c == y)) {
                    continue;
                }
                lookup[c != x] = 1;
                --remain[c != x];
                curr += (c == x) ? 1 : -1;
                if (curr < 0 && remain[0] && remain[1]) {
                    curr = lookup[0] = lookup[1] = 0;  // reset states if the remain has both x, y
                }
                if (lookup[0] && lookup[1]) {
                    result = max(result, curr);  // update result if x, y both exist
                }
            }
            return result;
        };
        
        unordered_set<char> alphabets(cbegin(s), cend(s));
        int result = 0;
        for (const auto& x : alphabets) {
            for (const auto& y: alphabets) {
                if (x != y) {
                    result = max(result, modified_kadane(x, y));
                }
            }
        }
        return result;
    }
};