7. Простой стек.

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int[] stack = new int[1000000];
        int size = 0;
        String command;

        while (true) {
            command = sc.nextLine();
            if (command.contains("push")) {
                stack[size++] = Integer.parseInt(command.substring(5));
                System.out.println("ok");
            }
            if (command.contains("pop")) {
                System.out.println(stack[size - 1]);
                stack[size--] = 0;
            }
            if (command.contains("back")) {
                System.out.println(stack[size - 1]);
            }
            if (command.contains("size")) {
                System.out.println(size);
            }
            if (command.contains("clear")) {
                if (size == 0) {
                    System.out.println("ok");
                } else {
                    for (int i = 0; i < size; i++) {
                        stack[i] = 0;
                    }
                    size = 0;
                    System.out.println("ok");
                }
            }
            if (command.contains("exit")) {
                System.out.println("bye");
                break;
            }
        }
    }
}