Seat Booking System

/*
 * Problem : Process lines.
 * Input   : Several command lines.
 * Output  : One result per command.
 * Approach: Theater class with a grid.
 * Time    : O(n * R * C)
 * Space   : O(R * C)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The theater owns a grid of booked flags. The outside world only uses book(), cancel(), free() and first().
        static class Theater {
            private final boolean[][] booked;
            private final int rows;
            private final int columns;

            Theater(int rows, int columns) {
                this.rows = rows;
                this.columns = columns;
                this.booked = new boolean[rows][columns];
            }

            private boolean inside(int r, int c) {
                return r >= 1 && r <= rows && c >= 1 && c <= columns;
            }

            // Step 2: Each method checks that the seat exists before it looks at the flag.
            String book(int r, int c) {
                if (!inside(r, c)) {
                    return "invalid";
                }
                if (booked[r - 1][c - 1]) {
                    return "taken";
                }
                booked[r - 1][c - 1] = true;
                return "ok";
            }

            String cancel(int r, int c) {
                if (!inside(r, c)) {
                    return "invalid";
                }
                if (!booked[r - 1][c - 1]) {
                    return "free";
                }
                booked[r - 1][c - 1] = false;
                return "ok";
            }

            int free() {
                int count = 0;
                for (boolean[] row : booked) {
                    for (boolean flag : row) {
                        if (!flag) {
                            count++;
                        }
                    }
                }
                return count;
            }

            String first() {
                for (int r = 0; r < rows; r++) {
                    for (int c = 0; c < columns; c++) {
                        if (!booked[r][c]) {
                            return (r + 1) + " " + (c + 1);
                        }
                    }
                }
                return "full";
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        String[] size = lines[0].split(" ");
        Theater theater = new Theater(Integer.parseInt(size[0]), Integer.parseInt(size[1]));
        String[] output = new String[lines.length - 1];
        for (int i = 1; i < lines.length; i++) {
            String[] part = lines[i].split(" ");
            if (part[0].equals("BOOK")) {
                output[i - 1] = theater.book(Integer.parseInt(part[1]), Integer.parseInt(part[2]));
            } else if (part[0].equals("CANCEL")) {
                output[i - 1] = theater.cancel(Integer.parseInt(part[1]), Integer.parseInt(part[2]));
            } else if (part[0].equals("FREE")) {
                output[i - 1] = String.valueOf(theater.free());
            } else {
                output[i - 1] = theater.first();
            }
        }
        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)));
    }
}