[스택] 백준 4949 균형잡힌 세상

https://www.acmicpc.net/problem/4949

#include <iostream>
#include <stack>
using namespace std;
 
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    
    while(1){
        string s;
        stack<char> st;
        bool chk = true; //yes or no
        
        getline(cin, s);
        
        if(s[0] == '.') break;
        
        int len = s.length();
        
        for(int i = 0; i < len; i++){
            if(s[i] == '(' || s[i] == '[') st.push(s[i]);
            else if(s[i] == ')'){
                if(st.empty() || st.top() == '['){
                    chk = false;
                    break;
                }
                else st.pop();
            }
            else if(s[i] == ']'){
                if(st.empty() || st.top() == '('){
                    chk = false;
                    break;
                }
                else st.pop();
            }
        }
        
        if(chk && st.empty()) cout << "yes" << endl;
        else cout << "no" << endl;
    }
    
}