32. Longest Valid Parentheses

class Solution:
    def longestValidParentheses(self, s):
        """
        :type s: str
        :rtype: int
        """
        maxans = 0
        st = []
        st.append(-1)
        for i in range(0, len(s)):
            if s[i] == '(':
                st.append(i)
            else:
                st.pop()
                if not st:
                    st.append(i)
                else:
                    maxans = max(maxans, i - st[-1])
        return maxans