13.8. Самое частое слово.

import java.util.Arrays;
import java.util.HashSet;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        HashSet<String> set = new HashSet<>();
        String s = sc.nextLine();
        String[] arr = s.split(" ");
        for (int i = 0; i < arr.length; i++) {
            for (int j = i + 1; j < arr.length; j++) {
                if (arr[i].equals(arr[j]))
                    set.add(arr[i]);
            }
        }
        Object[] out = set.toArray();
        Arrays.sort(out);
        System.out.println(out[0]);
    }
}