LeetCode.678 Valid Parenthesis String

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

2.Idea
Greedy approach. Evaluating possible lower and upper bound value of open parenthesis number.

3.Source

 class Solution {  
 public:  
   bool checkValidString(string s) {  
     int lo = 0, hi = 0;  
     for (int i = 0; i < s.size(); i++) {  
       lo += s[i] == '(' ? 1 : -1;  
       hi += s[i] != ')' ? 1 : -1;  
       if (hi < 0) break;  
       lo = std::max(lo, 0);  
     }  
     return lo == 0;  
   }  
 };