N28278 - 스택 2

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.BufferedWriter;
import java.io.OutputStreamWriter;
import java.io.IOException;
import java.util.Stack;
import java.util.Queue;
import java.util.LinkedList;


public class Main {
    private static final BufferedReader BR = new BufferedReader(new InputStreamReader(System.in));
    private static final BufferedWriter BW = new BufferedWriter(new OutputStreamWriter(System.out));
    private static final StringBuffer SB = new StringBuffer();

    public static void main(String[] args) throws IOException {
        Stack<Integer> stack = new Stack<>();
        Queue<String> instructions = new LinkedList<>();


        input(instructions);
        solution(stack, instructions);

        BW.write(SB.toString());
        BW.close();
        BR.close();
    }

    private static void input(Queue<String> instructions) throws IOException {
        int n = Integer.parseInt(BR.readLine());
        for (int i=0; i<n; i++)
            instructions.offer(BR.readLine());
    }

    private static void solution(Stack<Integer> st, Queue<String> instructions) throws IOException {
        while(instructions.size() != 0) {
            String s = instructions.poll();
            switch (s.charAt(0)) {
                case '1':
                    st.add(Integer.parseInt(s.substring(2)));
                    break;
                case '2':
                    if (st.size() == 0) SB.append(-1);
                    else SB.append(st.pop());
                    SB.append("\n");
                    break;
                case '3':
                    SB.append(st.size()).append("\n");
                    break;
                case '4':
                    if (st.size() == 0) SB.append(1);
                    else SB.append(0);
                    SB.append("\n");
                    break;
                case '5':
                    if (st.size() == 0) SB.append(-1);
                    else SB.append(st.peek());
                    SB.append("\n");
                    break;
            }
        }
    }
}