백준 #4949 - 균형잡힌 세상 C++

#include <iostream>
#include <stack>
#include <string>
using namespace std;
string balance(string s) {
    stack<char> st;
    char cur = ' ';
    for(int i = 0 ; i <s.length(); i++)
    {
        if (st.empty() && (s[i]== ')' || s[i] == ']')) {
            return "no";
        }
        if (s[i] == '(' || s[i] == '[') {
            st.push(s[i]);
            cur = s[i];
        }
        if ((cur == '(' && s[i] == ']') || (cur == '[' && s[i] == ')')) {
            return "no";
        }
        if ((cur == '(' && s[i] == ')') || (cur == '[' && s[i]==']')) {
            st.pop();
            if(!st.empty())
                cur = st.top();
        }
    }
    if (st.empty())
        return "yes";
    else
        return "no";
}
int main() {

    string s;

    while (true) {

        cin.clear();
        getline(cin, s);
        if (s == ".")
            break;
        cout << balance(s) << endl;
    }
    return 0;
}