这样写简单些！

class Solution {
public:
    bool isValid(string s) {
        stack<char> st;
        map<char, char> mp;
        mp[')'] = '(', mp['}'] = '{', mp[']'] = '[';
        for(char c: s){
            if(c == '(' || c == '{' || c == '[') st.push(c);
            else{
                if(st.empty() || st.top() != mp[c]) return false;
                st.pop();
            }
        }
        return st.empty();
    }
};