Shopping Cart Checkout

/*
 * Problem : Process lines.
 * Input   : Several command lines.
 * Output  : One result per command.
 * Approach: Cart class.
 * Time    : O(n * items)
 * Space   : O(items)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 1: The cart keeps its items and the coupon. The rules for the money are in one place.
        static class Cart {
            private final java.util.Map<String, long[]> items = new java.util.LinkedHashMap<String, long[]>();
            private long percent = 0;

            void add(String name, long price, long qty) {
                long[] item = items.get(name);
                if (item == null) {
                    items.put(name, new long[] {price, qty});
                } else {
                    item[1] += qty;
                }
            }

            boolean remove(String name, long qty) {
                long[] item = items.get(name);
                if (item == null || item[1] < qty) {
                    return false;
                }
                item[1] -= qty;
                if (item[1] == 0) {
                    items.remove(name);
                }
                return true;
            }

            void coupon(long percent) {
                this.percent = percent;
            }

            // Step 2: Subtotal, then discount rounded down, then total.
            String total() {
                long subtotal = 0;
                for (long[] item : items.values()) {
                    subtotal += item[0] * item[1];
                }
                long discount = subtotal * percent / 100;
                return subtotal + " " + discount + " " + (subtotal - discount);
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        Cart cart = new Cart();
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] part = lines[i].split(" ");
            if (part[0].equals("ADD")) {
                cart.add(part[1], Long.parseLong(part[2]), Long.parseLong(part[3]));
                output[i] = "ok";
            } else if (part[0].equals("REMOVE")) {
                output[i] = cart.remove(part[1], Long.parseLong(part[2])) ? "ok" : "error";
            } else if (part[0].equals("COUPON")) {
                cart.coupon(Long.parseLong(part[1]));
                output[i] = "ok";
            } else {
                output[i] = cart.total();
            }
        }
        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)));
    }
}