32. Longest Valid Parentheses leetcode python new season 2016

Given a string containing just the characters 

'('

 and 

')'

,
find the length of the longest valid (well-formed) parentheses substring.

For 

"(()"

, the longest valid parentheses substring is 

"()"

,
which has length = 2.

Another example is 

")()())"

, where the longest valid parentheses substring is 

"()()"

,
which has length = 4.

class Solution(object):
def longestValidParentheses(self, s):
"""
:type s: str
:rtype: int
"""
s = ')' + s
stack, result = [], 0
for index in range(len(s)):
element = s[index]
if element == ')' and stack and stack[-1][1] == '(':
stack.pop()
result = max(result, index - stack[-1][0])
else:
stack.append((index, s[index]))
return result