백준 10828번(Java)

import java.io.*;
import java.util.*;

public class Main {

	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringBuilder sb = new StringBuilder();
		
		int N = Integer.parseInt(br.readLine());
		
		Stack<Integer> stack = new Stack<>();
		
		for(int i=0; i<N; i++) {
			StringTokenizer st = new StringTokenizer(br.readLine());
			String s = st.nextToken();
			
			switch (s) {
		    case "push":
		        stack.push(Integer.parseInt(st.nextToken()));
		        break;
		    case "pop":
		        sb.append(stack.empty() ? -1 : stack.pop()).append("\n");
		        break;
		    case "size":
		        sb.append(stack.size()).append("\n");
		        break;
		    case "empty":
		        sb.append(stack.empty() ? 1 : 0).append("\n");
		        break;
		    case "top":
		        sb.append(stack.empty() ? -1 : stack.peek()).append("\n");
		        break;
			}
		}
		System.out.println(sb);
	}
}