Longest Balanced Substring

• Time : O(n)
• Space : O(n)

int longestBalancedSubstring(string str) {
    vector<int> st{-1};
    int n = str.length(), res = 0;
    for(int i = 0; i < n; i++) {
        if(str[i] == '(') st.push_back(i);
        else {
            if(st.back() == -1 or str[st.back()] == ')') st.push_back(i);
            else {
                st.pop_back();
                res = max(res, i - st.back());
            }
        }
    }
    return res;
}