10.1.17. Дек.

import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        LinkedList<Integer> deque = new LinkedList<>();
        int size = 0;
        String command;

        while (true) {
            command = sc.nextLine();
            if (command.contains("push_front")) {
                deque.addFirst(Integer.parseInt(command.substring(11)));
                size++;
                System.out.println("ok");
            }
            if (command.contains("push_back")) {
                deque.addLast(Integer.parseInt(command.substring(10)));
                size++;
                System.out.println("ok");
            }
            if (command.equals("pop_front")) {
                if (size <= 0) {
                    System.out.println("error");
                } else {
                    System.out.println(deque.removeFirst());
                    size--;
                }
            }
            if (command.equals("pop_back")) {
                if (size <= 0) {
                    System.out.println("error");
                } else {
                    System.out.println(deque.removeLast());
                    size--;
                }
            }
            if (command.equals("front")) {
                if (size <= 0) {
                    System.out.println("error");
                } else {
                    System.out.println(deque.getFirst());
                }
            }
            if (command.equals("back")) {
                if (size <= 0) {
                    System.out.println("error");
                } else {
                    System.out.println(deque.getLast());
                }
            }
            if (command.equals("size")) {
                System.out.println(size);
            }
            if (command.equals("clear")) {
                if (size == 0) {
                    System.out.println("ok");
                } else {
                    for (int i = 0; i < size; i++) {
                        deque.removeFirst();
                    }
                    size = 0;
                    System.out.println("ok");
                }
            }
            if (command.contains("exit")) {
                System.out.println("bye");
                break;
            }
        }
    }
}