chatgpt wala

string getLongestRegex(string a, string b, string c) {
    int n = a.size();

    vector<set<char>> candidates(n, set<char>());

    for (int i = 0; i < n; i++) {
        candidates[i].insert(a[i]);
        candidates[i].insert(b[i]);
    }

    for (int i = 0; i < n; i++) {
        if (a[i] != c[i]) {
            candidates[i].erase(a[i]);
        }
        if (b[i] != c[i]) {
            candidates[i].erase(b[i]);
        }
    }

    string regex = "";
    for (int i = 0; i < n; i++) {
        if (candidates[i].empty()) {
            return "-1";
        }
        if (candidates[i].size() == 1) {
            regex += "[" + string(1, *candidates[i].begin()) + "]";
        } else {
            regex += "[";
            for (char ch : candidates[i]) {
                regex += ch;
            }
            regex += "]";
        }
    }

    return regex;
}