[LeetCode] (medium) 678. Valid Parenthesis String

博客围绕LeetCode上验证含'(', ')'和'*'三种字符的字符串是否有效的问题展开。介绍了字符串有效的规则，星号可当作右括号、左括号或空字符串。还提到星号使未成匹的左括号个数成范围，且范围左端点不能小于零。

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:

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.
An empty string is also valid.

Example 1:
Input: "()"
Output: True
Example 2:
Input: "(*)"
Output: True
Example 3:
Input: "(*))"
Output: True
Note:

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

https://leetcode.com/problems/valid-parenthesis-string/discuss/107611/Very-concise-C%2B%2B-solution-with-explaination.-No-DP

星号的引入使得未成匹的左括号个数成为了一个范围

注意范围左端点不能小于零

class Solution {
public:
    bool checkValidString(string s) {
        int lo = 0, hi = 0;
        for(int i = 0; i < s.size(); ++i){
            if(s[i] == '('){
                ++lo;
                ++hi;
            }else if(s[i] == ')'){
                --lo;
                --hi;
            }else{
                --lo;
                ++hi;
            }
            if(lo < 0) lo = 0;
            if(hi < 0) return false;
        }
        
        return lo == 0;
    }
};