20. Valid Parentheses

class Solution {
public:
    bool isValid(string s) {
        stack<char> tmp;
        for (int i = 0; i < s.size(); i++) {
            if (s[i] == '{' || s[i] == '(' || s[i] == '[')
                tmp.push(s[i]);
                else {
                    if (tmp.empty()) return false;    //pay attention !!!
                    if (s[i] == '}' && tmp.top() != '{') return false;
                    if (s[i] == ')' && tmp.top() != '(') return false;
                    if (s[i] == ']' && tmp.top() != '[') return false;
                    tmp.pop();
                }
        }
        return tmp.empty();    //notice if it's not empty should also return false!!!
    }
};