20. 有效的括号

//思路很清晰，直接用stack
class Solution 
{
    unordered_map<char,char> hash = {{'(',')'},{'[',']'},{'{','}'}};
public:
    bool isValid(string s) 
    {
        stack<char> stk;
        for(auto a : s)
        {
            if(a == '(' || a == '{' || a == '[') stk.push(a);
            else
            {
                if(!stk.empty() && a == hash[stk.top()]) stk.pop();
                else return false;
            }
        }
        return stk.size() == 0;
    }
};