[백준]4949-균형잡힌 세상(설명X)

#include <cstdio>
#include <stack>
#include <iostream>
#include <cstring>
using namespace std;
 
int main()
{
    while (true) {
        stack<char> stk;
        char str[100];
        scanf("%[^\n]", str);
        int size = strlen(str);
        cin.ignore();
        if (str[0] == '.' && size == 1)
            break;
        bool equal = true;
        for (size_t i = 0; i < size; i++) {
            if (str[i] == '(')
                stk.push('(');
            else if (str[i] == '[')
                stk.push('[');
            else if (str[i] == ')') {
                if (stk.size() == 0 || stk.top() != '(')
                    equal = false;
                else
                    stk.pop();
            }
            else if (str[i] == ']') {
                if (stk.size() == 0 || stk.top() != '[')
                    equal = false;
                else
                    stk.pop();
            }
            else if (str[i] == '.')
                break;
        }
        if (equal && stk.size() == 0)
            printf("yes\n");
        else
            printf("no\n");
    }
    return 0;
}