discrete.java

import java.util.Random;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.List;

public class discrete {
  public static void main(String[] args) {
    String input = "ABCDEF";
    List<String> permutations = generatePermutations(input);
    for (String perm : permutations) {
      System.out.println(perm);
    }
  }

  public static List<String> generatePermutations(String input) {
    List<String> permutations = new ArrayList<>();
    boolean[] used = new boolean[input.length()];
    StringBuilder current = new StringBuilder();
    generatePermutationsHelper(input, current, used, permutations);
    return permutations;
  }


  private static void generatePermutationsHelper(String input, StringBuilder current, boolean[] used,
      List<String> permutations) {
    if (current.length() == input.length()) {
      permutations.add(current.toString());
      return;
    }
    for (int i = 0; i < input.length(); i++) {
      if (!used[i]) {
        current.append(input.charAt(i));
        used[i] = true;
        generatePermutationsHelper(input, current, used, permutations);
        current.deleteCharAt(current.length() - 1);
        used[i] = false;
        while (i + 1 < input.length() && input.charAt(i) == input.charAt(i + 1)) {
          i++;
        }
      }
    }
  }
}

/*
 * public static void main(String[] args) {
 * String[] letters = { "A", "B", "C", "D", "E", "F" };
 * String[] permutations = new String[6];
 * 
 * 
 * 
 * 
 * 
 * for(int i = 0; i<letters.length; i++) {
 * int counter = 0;
 * int ext = 0;
 * char c = letters[i].charAt(i);
 * for(int j =0;j<i;j++){
 * if(c==letters[j].charAt(j)){
 * counter++;
 * }
 * }
 * if(counter==1){
 * ext += 1;
 * }
 * }
 * 
 * }
 */