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 '*'.

Complexity

• Time complexity: O(n)

• Space complexity: O(1)

Code

class Solution:
    def checkValidString(self, s: str) -> bool:
        cmin = cmax = 0
        for ch in s:
            if ch == '(':
                cmax += 1
                cmin += 1
            if ch == ')':
                cmax -= 1
                cmin = max(cmin - 1, 0)
            if ch == '*':
                cmax += 1
                cmin = max(cmin - 1, 0)
            if cmax < 0:
                return False
        return cmin == 0