MinUniqueCharacters.java

import java.util.*;

public class MinUniqueCharacters {

    public static String[] findWordWithMinUniqueChars(String inputString) {
        String[] words = inputString.split("\\s+");

        String resultWord = "";
        int minUniqueChars = Integer.MAX_VALUE;

        for (String word : words) {
            int uniqueCharsCount = countUniqueCharacters(word);

            if (uniqueCharsCount < minUniqueChars) {
                minUniqueChars = uniqueCharsCount;
                resultWord = word;
            }
        }

        return new String[]{resultWord};
    }

    private static int countUniqueCharacters(String word) {
        Set<Character> uniqueChars = new HashSet<>();
        for (char c : word.toCharArray()) {
            uniqueChars.add(c);
        }
        return uniqueChars.size();
    }

    public static void main(String[] args) {
        String inputString = "apple aaaaaaa banana aaaaa abc a abcde";

        String[] result = findWordWithMinUniqueChars(inputString);

        System.out.println(Arrays.toString(result));
    }
}