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