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Valid Parenthesis String

Problem Statement​

Given a string s containing only three types of characters: '(', ')' and '*', return true if s is valid.

The following rules define a valid string:

  • Any left parenthesis '(' must have a corresponding right parenthesis ')'.
  • Any right parenthesis ')' must have a corresponding left parenthesis '('.
  • Left parenthesis '(' must go before the corresponding right parenthesis ')'.
  • '*' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string "".

Leetcode Link

Example 1:​

Input: s = "()"
Output: true

Example 2:​

Input: s = "(*)"
Output: true

Example 3:​

Input: s = "(*))"
Output: true

Constraints:​

  • 1 <= s.length <= 100
  • s[i] is '(', ')' or '*'.

Code​

Python Code
class Solution:
        def checkValidString(self, s: str) -> bool:
        stack, star = [], []

        for i,char in enumerate(s):
            if char == "(": stack.append(i)
            elif char == ")":
                if not stack and not star: return False
                elif stack: stack.pop()
                elif star: star.pop()
            else: star.append(i)

        while stack:
            if star and stack[-1] < star[-1]:
                stack.pop()
                star.pop()
            else:
                if not star: return False
                else: star.pop()

        return True