# Valid Parenthesis String Problem

## Description

LeetCode Problem 678.

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:

``````1
2
Input: s = "()"
Output: true
``````

Example 2:

``````1
2
Input: s = "(*)"
Output: true
``````

Example 3:

``````1
2
Input: s = "(*))"
Output: true
``````

Constraints:

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

## Sample C++ Code

``````1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Solution {
public:
bool checkValidString(string s) {
int lower = 0, upper = 0;
for (char c : s) {
if (c == '(') {
lower ++;
upper ++;
} else if (c == ')') {
lower --;
upper --;
} else {
// * encountered
lower --;
upper ++;
}
lower = max(lower, 0);
if (upper < 0)
// unmatched ')' found in the middle of string
return false;
}
return lower == 0;
}
};
``````