Program to find length of longest valid parenthesis from given string in Python

PythonServer Side ProgrammingProgramming

Suppose we have a string s. This s consists of opening and closing parenthesis only. We have to find the length of the longest valid (well-formed) parentheses substring. So if the input is like “))(())())”, then the result will be 6, as the valid string is “(())()”.

To solve this, we will follow these steps −

  • Make a stack, and insert -1., set ans := 0

  • for i in range 0 to length of stack – 1

    • if s[i] is opening parentheses, then insert i into stack

    • otherwise

      • if stack is not empty and top of stack is not -1 and s[stack top] is opening parentheses, then

        • top element from stack

        • ans := max of ans and i – stack top

      • otherwise insert i into stack

  • return ans


Let us see the following implementation to get a better understanding −

 Live Demo

class Solution(object):
   def solve(self, s):
      stack = [-1]
      ans = 0
      for i in range(len(s)):
         if s[i] == "(":
            if stack and stack[-1]!=-1 and s[stack[-1]] == "(":
               ans = max(ans,i - stack[-1])
      return ans
ob = Solution()




Updated on 22-Dec-2020 06:27:58