class Solution {
public:
    bool checkValidString(string s) {
        int len = s.size(), lo = 0, hi = 0;//range of # of open left bracket
        for(int i = 0; i < len; ++i)
        {
            lo = s[i] == '('? lo + 1: lo - 1;
            hi = s[i] == ')'? hi - 1: hi + 1;
            if(hi < 0)return false;
            lo = max(lo, 0);
        }
        return lo == 0;
    }
};