백준 10828번 스택

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Stack;
import java.util.StringTokenizer;

public class Main {
    public static void main(String[] args) throws IOException {
        
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        Stack<Integer> stack = new Stack<>();
        StringBuilder sb = new StringBuilder();
        
        for (int i = 0; i < n; i++) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            String m = st.nextToken();
            
            if (m.equals("push")) {
                int number = Integer.parseInt(st.nextToken());
                stack.push(number);
            } else if (m.equals("pop")) {
                if (!stack.isEmpty()) {
                    sb.append(stack.pop()).append('\n');
                } else {
                    sb.append("-1").append('\n');
                }
            } else if (m.equals("size")) {
                sb.append(stack.size()).append('\n');
            } else if (m.equals("empty")) {
                sb.append(stack.isEmpty() ? "1" : "0").append('\n');
            } else if (m.equals("top")) { 
                if (!stack.isEmpty()) {
                    sb.append(stack.peek()).append('\n');
                } else {
                    sb.append("-1").append('\n');
                }
            }
        }
        
        System.out.println(sb.toString());
    }
}