Permutations

permutations.java

import java.util.HashSet;

public class Permutations {
  public static HashSet<String> permutations(String str) { 
    return permutations("", str); 
  }

  private static HashSet<String> permutations(String prefix, String str) {
    HashSet<String> result = new HashSet<>();
    
    int n = str.length();
    if (n == 1) result.add(prefix + str);
    else {
        for (int i = 0; i < n; i++)
            result.addAll(permutations(prefix + str.charAt(i), str.substring(0, i) + str.substring(i + 1)));
    }
    
    return result;
  }

  public static void main(String[] args) {
    permutations("abba").forEach(System.out::println);
  }
}