줄을 지어 순서대로 처리되는 것
자바에서 큐는 LinkedList 활용
package baekjoon;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.Queue;
import java.util.StringTokenizer;
public class Queue_10845 {
public static void main(String args[]) throws IOException {
Queue<Integer> queue=new LinkedList<>();
BufferedReader bf=new BufferedReader(new InputStreamReader(System.in));
int N=Integer.parseInt(bf.readLine());
int b=0;
for(int i=0;i<N;i++) {
StringTokenizer st=new StringTokenizer(bf.readLine());
String S=st.nextToken();
switch(S){
case "push":
int num=Integer.parseInt(st.nextToken());
queue.add(num);
b=num;
break;
case "pop":
if(queue.isEmpty()) {
System.out.println(-1);
} else {
System.out.println(queue.poll());
}
break;
case "size":
System.out.println(queue.size());
break;
case "empty":
if(queue.isEmpty()){
System.out.println(1);
}
else {
System.out.println(0);
}
break;
case "front":
if(queue.isEmpty()) {
System.out.println(-1);
} else {
System.out.println(queue.peek());
}
break;
case "back":
if(queue.isEmpty()) {
System.out.println(-1);
} else {
System.out.println(b);
}
break;
}
}
}
}