Leetcode 678. Valid Parenthesis String

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 "".

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 '*'.

This question can be solved by using stack.

The idea is pairing valid parentheses first, then check if star sign can make up those invalid parentheses.

class Solution:
    def checkValidString(self, s: str) -> bool:
        star_positions = []
        open_parentheses = []

        for i in range(len(s)):
            if s[i] == '(':
                open_parentheses.append(i)
            elif s[i] == '*':
                star_positions.append(i)
            else:
                if open_parentheses:
                    open_parentheses.pop()
                elif star_positions:
                    star_positions.pop()
                else:
                    return False
        
        while open_parentheses and star_positions:
            open_index = open_parentheses.pop()
            star_index = star_positions.pop()

            if star_index < open_index:
                return False
        
        return False if open_parentheses else True