1. 올바른 괄호

import java.util.*;

class Main {
	public String solution(String s) {
		String answer = "YES";
		int cnt = 0;
		for (char c : s.toCharArray()) {
			if (c == '(')
				cnt++;
			else {
				cnt--;
				if (cnt < 0)
					return "NO";
			}
		}
		if (cnt != 0)
			answer = "NO";
		return answer;
	}

	public static void main(String[] args) {
		Main T = new Main();
		Scanner kb = new Scanner(System.in);
		String s = kb.next();
		System.out.print(T.solution(s));
		kb.close();
	}
}

 

import java.util.*;

class Main {
	public String solution(String s) {
		String answer = "YES";
		Stack<Character> stack = new Stack<>();
		for (char c : s.toCharArray()) {
			if (c == '(')
				stack.push(c);
			else {
				if (stack.empty())
					return "NO";
				stack.pop();
			}
		}
		if (!stack.isEmpty())
			return "NO";
		return answer;
	}

	public static void main(String[] args) {
		Main T = new Main();
		Scanner kb = new Scanner(System.in);
		String s = kb.next();
		System.out.print(T.solution(s));
		kb.close();
	}
}

Stack 사용