백준 9012번 괄호

https://www.acmicpc.net/problem/9012

#include <iostream>
#include <cstring>
#include <stack>
#pragma warning(disable:4996)
using namespace std;
int main(void) {
	int test_case = 0;
	char str[51] = { 0, };
	scanf("%d\n", &test_case);
	for (int i = 0; i < test_case; ++i)
	{
		scanf("%s", &str);
		int len = strlen(str);
		bool flag = false;
		stack<char> s;
		for (int j = 0; j < len; ++j)
		{
			if (str[j] == '(')
				s.push(str[j]);
			else
			{
				if (s.empty())
				{
					flag = true;
					break;
				}
				s.pop();
			}
		}
		if (flag || !s.empty())
			printf("NO\n");
		else
			printf("YES\n");
	}
	return 0;
}