678. Valid Parenthesis String

678. Valid Parenthesis String

Medium

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 '*'.

Solution:

class Solution {

    /**
     * @param String $s
     * @return Boolean
     */
    function checkValidString($s) {
        $lower = 0;
        $upper = 0;
        for ($i = 0; $i < strlen($s); $i++) {
            $lower += ($s[$i] == '(') ? 1 : -1;
            $upper -= ($s[$i] == ')') ? 1 : -1;
            if ($upper < 0) break;
            $lower = max($lower, 0);
        }
        return $lower == 0;
    }
}

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