32. Longest Valid Parentheses

Given a string containing just the characters ‘(‘ and ‘)’, find the length of the longest valid (well-formed) parentheses substring.

class Solution {
public:
    int longestValidParentheses(string s) {
        vector<int> st {-1};
        int res = 0;
        for(int i = 0; i < s.length(); i++) {
            if(s[i] == '(') {
                st.push_back(i);
            } else {
                st.pop_back();
                if(st.empty()) st.push_back(i);
                else res = max(res, i - st.back());
            }
        }
        return res;
    }
};