class Solution(object):

    def longestValidParentheses(self, s):

        """

        :type s: str

        :rtype: int

        """

        maxlen=0

        stack=[]

        last=-1

        for i in range(len(s)):

            if s[i] == '(':

                stack.append(i)

            else:

                if stack == []:

                    last=i

                else:

                    stack.pop()

                    if stack == []:

                        maxlen=max(maxlen,i-last)

                    else:

                        maxlen=max(maxlen,i-stack[len(stack)-1])

        return maxlen