[leetcode]Longest Valid Parentheses @ Python

原题地址：https://oj.leetcode.com/problems/longest-valid-parentheses/

题意：

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.

解题思路：返回括号串中合法括号串的长度。使用栈。这个解法比较巧妙，开辟一个栈，压栈的不是括号，而是未匹配左括号的索引！

代码：

class Solution:
    # @param s, a string
    # @return an integer
    def longestValidParentheses(self, s):
        maxlen = 0
        stack = []
        last = -1
        for i in range(len(s)):
            if s[i]=='(':
                stack.append(i)     # push the INDEX into the stack!!!!
            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