[leetcode] Longest Valid Parentheses @python

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.

Solution 1: This solution is more natural and easy to understand.

class Solution:
    # @param s, a string
    # @return an integer
    def longestValidParentheses(self, s):        
        stack = [(-1, ')')]
        maxLen = 0
        for i in range(len(s)):
            if s[i] == ')' and stack[-1][1] == '('
                stack.pop()
                maxLen = max(maxLen, i - stack[-1][0])
            else:
                stack.append( (i, s[i]) )
        return maxLen

 

 

Solution 2: It is smart but not practical

 

class Solution:
    # @param s, a string
    # @return an integer
    def longestValidParentheses(self, s):
        #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！
        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[-1])
        return maxLen

 参考致谢:

[1] http://sumnous.github.io/blog/2014/05/10/leetcode-longest-valid-parentheses-at-python/

[2] http://www.cnblogs.com/zuoyuan/p/3780312.html