LeetCode 678. Valid Parenthesis String

Description

https://leetcode.com/problems/valid-parenthesis-string/

Given a string containing only three types of characters: ‘(‘, ‘)’ and ‘*’, write a function to check whether this string is valid. We define the validity of a string by these rules:

  1. Any left parenthesis '(' must have a corresponding right parenthesis ')'.
  2. Any right parenthesis ')' must have a corresponding left parenthesis '('.
  3. Left parenthesis '(' must go before the corresponding right parenthesis ')'.
  4. '*' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string.
  5. An empty string is also valid.

Example 1:

Input: "()"
Output: True

Example 2:

Input: "(*)"
Output: True

Example 3:

Input: "(*))"
Output: True

Note:

  1. The string size will be in the range [1, 100].

Explanation

two stacks one stores ‘(‘, one stores ‘*’. If encounter ‘)’, check if there still any ‘(‘ exist, it not, check if ‘*’ exists. If string like “*(“, then not valid.

Python Solution

class Solution:
    def checkValidString(self, s: str) -> bool:
        left = []
        star = []
        
        for i, ch in enumerate(s):
            if ch == '*':
                star.append(i)
            elif ch == '(':
                left.append(i)
            else:
                if len(left) == 0 and len(star) == 0:
                    return False
                if len(left) > 0:
                    left.pop()
                else:                
                    star.pop()
                    
        while (len(left) > 0 and len(star) > 0):
            if (left[-1] > star[-1]):
                return False
            left.pop()
            star.pop()
                
        return len(left) == 0
  • Time complexity: O(N).
  • Space complexity: O(N).

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