Multiset with Counts

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

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String[] solve(String[] lines) {
        // Step 1: A map from item to its number of copies. An item with 0 copies is removed from the map, so DISTINCT is the map size.
        java.util.Map<String, Integer> counts = new java.util.HashMap<String, Integer>();
        long total = 0;
        String[] output = new String[lines.length];
        for (int i = 0; i < lines.length; i++) {
            String[] part = lines[i].split(" ");
            if (part[0].equals("ADD")) {
                counts.merge(part[1], 1, Integer::sum);
                total++;
                output[i] = "ok";
            } else if (part[0].equals("REMOVE")) {
                Integer have = counts.get(part[1]);
                if (have == null) {
                    output[i] = "none";
                } else {
                    // Step 2: Take one copy. Drop the item when the last copy is gone.
                    if (have == 1) {
                        counts.remove(part[1]);
                    } else {
                        counts.put(part[1], have - 1);
                    }
                    total--;
                    output[i] = "ok";
                }
            } else if (part[0].equals("COUNT")) {
                output[i] = String.valueOf(counts.getOrDefault(part[1], 0));
            } else if (part[0].equals("DISTINCT")) {
                output[i] = String.valueOf(counts.size());
            } else {
                output[i] = String.valueOf(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)));
    }
}