Leetcode20. 有效的括號

class Solution {
public:
    bool isValid(string s) {
        int n = s.size();
        if (n%2==1){
            return false;
        }
        unordered_map<char, char> mymap={{')', '('}, {'}', '{'}, {']', '['}};
        stack<char> st;
        for (char ch:s){
            if (mymap.count(ch)){
                if (st.empty() || st.top() != mymap[ch]){
                    return false;
                }
                st.pop();
            }
            else{
                st.push(ch);
            }
        }
        return st.empty();
    }
};