Letter Tile Possibilities

import java.util.HashSet;
import java.util.Set;

/**
 * Created by bugkiller on 09/06/19.
 */
class LetterTilePossibilities {

    public static void main(String[] args) {
        LetterTilePossibilities obj = new LetterTilePossibilities();
        System.out.println(obj.numTilePossibilities("ABCDEF"));
    }

    public int numTilePossibilities(String tiles) {
        Set<String> result = new HashSet<>();
        possibilities(tiles, "", result);
        return result.size();
    }

    private void possibilities(String rem, String pre, Set<String> result) {
        if (rem.length() == 0) {
            if (pre.length() != 0) {
                result.add(pre);
            }
            return;
        }
        for (int i = 0; i < rem.length(); i++) {
            char c = rem.charAt(i);
            String before = rem.substring(0, i);
            String after = rem.substring(i + 1);
            String newRem = before + after;
            possibilities(newRem, pre, result);
            possibilities(newRem, pre + c, result);
        }
    }
}