https://www.hackerrank.com/challenges/java-string-compare/problem

Java TreeSet Example.java

import java.util.Scanner;

public class Solution {

    public static String getSmallestAndLargest(String str, int k) {
        String smallest = "";
        String largest = "";

        // Complete the function
        // 'smallest' must be the lexicographically smallest substring of length 'k'
        // 'largest' must be the lexicographically largest substring of length 'k'
        java.util.SortedSet<String> sets = new java.util.TreeSet<>();
        for (int i = 0; i <= str.length() - k; i++) {
            sets.add(str.substring(i, i + k));
        }

        smallest = sets.first();
        largest = sets.last();
        return smallest + "\n" + largest;
    }

    public static void main(String[] args) {
        String s;
        int k;
        try (Scanner scan = new Scanner(System.in)) {
            s = scan.next();
            k = scan.nextInt();
        }

        System.out.println(getSmallestAndLargest(s, k));
    }
}