Find Duplicate Values

/*
 * Problem : Process an array.
 * Input   : n, then n integers.
 * Output  : The duplicate values.
 * Approach: Count the values.
 * Time    : O(n log 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(int[] a) {
        // Step 1: Count each value in a TreeMap (sorted by key).
        java.util.TreeMap<Integer, Integer> counts = new java.util.TreeMap<>();
        for (int value : a) {
            counts.merge(value, 1, Integer::sum);
        }

        // Step 2: Collect the values with a count above 1.
        StringBuilder text = new StringBuilder();
        for (java.util.Map.Entry<Integer, Integer> entry : counts.entrySet()) {
            if (entry.getValue() > 1) {
                text.append(text.length() == 0 ? "" : " ").append(entry.getKey());
            }
        }
        return text.length() == 0 ? "none" : text.toString();
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        System.out.println(solve(a));
    }
}