코드
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
public class BeakJoon10845 {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int n = Integer.parseInt(br.readLine());
List<Integer> list = new ArrayList<>();
StringTokenizer st;
for (int i = 0; i < n; i++) {
st = new StringTokenizer(br.readLine());
String temp = st.nextToken();
if ("push".equals(temp)) {
list.add(Integer.parseInt(st.nextToken()));
} else if ("pop".equals(temp)) {
if (list.isEmpty()) {
System.out.println("-1");
} else {
System.out.println(list.get(0));
list.remove(0);
}
} else if ("size".equals(temp)) {
System.out.println(list.size());
} else if ("empty".equals(temp)) {
if (list.isEmpty()) {
System.out.println("1");
} else {
System.out.println("0");
}
} else if ("front".equals(temp)) {
if (list.isEmpty()) {
System.out.println("-1");
} else {
System.out.println(list.get(0));
}
} else {
if (list.isEmpty()) {
System.out.println("-1");
} else {
System.out.println(list.get(list.size() - 1));
}
}
}
}
}