20. Valid Parentheses

https://leetcode.com/problems/valid-parentheses/description/

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