32. Longest Valid Parentheses（最长括号匹配，hard）

 

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.

code1:

()():返回2

class Solution:
    def longestValidParentheses(self, s):
        ans = 0
        stack = []
        for i,item in enumerate(s):
            if item == '(':
                stack.append(i)
            else:
                if(stack != []):
                    ans = max(ans,i-stack[-1]+1)
                    stack.pop()
        return ans

code2:

()():返回4

不是很理解

class Solution:
    def longestValidParentheses(self, s):
        ans = 0
        stack = []
        last =-1 
        for i,item in enumerate(s):
            if item == '(':
                stack.append(i)
            else:
                if(stack == []):#已经匹配成功了
                    last = i
                else:
                    stack.pop()
                    if stack==[]:
                        ans = max(ans,i-last)
                    else:
                        ans = max(ans,i-stack[-1])
        return ans