백준 10845번 큐 - JAVA

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

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());
    	Queue<Integer> q = new LinkedList<>();
    	BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
    	int back = -1;
    	for(int i = 0; i < n; i++) {
    		String temp = br.readLine();
    		if(temp.length() > 3 && temp.substring(0,4).equals("push")) {
    			back = Integer.parseInt(temp.substring(5,temp.length()));
    			q.offer(back);
    		}else if(temp.equals("pop")) {
    			if(!q.isEmpty()) bw.write(q.poll()+"\n");
    			else bw.write("-1\n");
    		}else if(temp.equals("size")) {
    			bw.write(q.size()+"\n");
    		}else if(temp.equals("empty")) {
    			if(q.isEmpty()) bw.write("1\n");
    			else bw.write("0\n");
    		}else if(temp.equals("front")) {
    			if(q.isEmpty()) bw.write("-1"+"\n");
    			else bw.write(q.peek()+"\n");
    		}else if(temp.equals("back")) {
    			if(q.isEmpty()) bw.write("-1"+"\n");
    			else bw.write(back+"\n");
    		}
    	}
    	bw.flush();
    	bw.close();  	
    }
}