Parking Lot System

/*
 * Problem : Process commands line by line.
 * Input   : Several lines of commands.
 * Output  : One line for each command.
 * Approach: A class that owns the slots.
 * Time    : O(commands * capacity)
 * Space   : O(capacity)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The parking lot class hides its data. The slots are stored in an array of car names. null means: the slot is free.
        static class ParkingLot {
            private final String[] slots;

            ParkingLot(int capacity) {
                slots = new String[capacity + 1];
            }

            String park(String car) {
                for (int i = 1; i < slots.length; i++) {
                    if (car.equals(slots[i])) {
                        return "Error: " + car + " already parked";
                    }
                }
                for (int i = 1; i < slots.length; i++) {
                    if (slots[i] == null) {
                        slots[i] = car;
                        return "Slot " + i;
                    }
                }
                return "Lot full";
            }

            String leave(String car) {
                for (int i = 1; i < slots.length; i++) {
                    if (car.equals(slots[i])) {
                        slots[i] = null;
                        return "Freed slot " + i;
                    }
                }
                return "Error: " + car + " not found";
            }

            int freeSlots() {
                int free = 0;
                for (int i = 1; i < slots.length; i++) {
                    if (slots[i] == null) {
                        free++;
                    }
                }
                return free;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        ParkingLot lot = new ParkingLot(Integer.parseInt(lines[0]));
        String[] output = new String[lines.length - 1];

        // Step 2: Run each command on the lot object.
        for (int i = 1; i < lines.length; i++) {
            String[] parts = lines[i].split(" ");
            if (parts[0].equals("park")) {
                output[i - 1] = lot.park(parts[1]);
            } else if (parts[0].equals("leave")) {
                output[i - 1] = lot.leave(parts[1]);
            } else {
                output[i - 1] = "Free: " + lot.freeSlots();
            }
        }
        return output;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(String.join("\n", solve(lines)));
    }
}