백준 큐2

• https://www.acmicpc.net/problem/18258

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


public class Main {

    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringBuilder sb = new StringBuilder();
        Deque<Integer> s = new LinkedList<>();
        String[] ins;

        int n = Integer.parseInt(br.readLine());

        for (int i = 0; i < n; i++) {

            ins = br.readLine().split(" ");

            if (ins[0].equals("push")) {
                s.add(Integer.parseInt(ins[1]));
            }

            //pop
            else if (ins[0].equals("pop")) {
                if (s.isEmpty()) {
                    sb.append(-1).append('\n');
                } else {
                    sb.append(s.remove()).append('\n');
                }
            }

            //size
            else if (ins[0].equals("size")) {
                sb.append(s.size()).append('\n');
            } else if (ins[0].equals("empty")) {
                if (s.isEmpty()) {
                    sb.append("1").append('\n');
                } else {
                    sb.append("0").append('\n');
                }

            }

            //back
            else if (ins[0].equals("top")) {
                if (s.isEmpty()) {
                    sb.append("-1").append('\n');
                } else {
                    sb.append(s.peek()).append('\n');
                }
            }

            else if(ins[0].equals("front")) {
                if (s.isEmpty()) {
                    sb.append(-1).append('\n');
                }
                else {
                    sb.append(s.peek()).append('\n');
                }
            }

            else if(ins[0].equals("back")) {
                if (s.isEmpty()) {
                    sb.append(-1).append('\n');
                }
                else {
                    sb.append(s.peekLast()).append('\n');
                }
            }

            else {
                System.out.println("ERROR");
            }

        }
        System.out.println(sb.toString());
        br.close();
        return;

    }

}