doanphihai

public class Rare {
    public static int nthMostRare(int[] elements, int n) {
        final Map<Integer, Integer> map = new HashMap<>();
        for (int i: elements)
        {
            map.compute(i, (k, v) -> {
                if (k == null) {
                    return 1;
                } else {
                    if (v == null) return 1;