Method numTilePossibilities

LetterTilePossibilities-1.java:4–10

(String tiles)

class Solution {
    int len;
    public int numTilePossibilities(String tiles) {
        len = tiles.length();
        boolean used[] = new boolean[len];
        HashSet<String> set = new HashSet<>();
        backtrack(tiles, used, set, "");
        return set.size() - 1;   
    }
    public void backtrack(String tiles, boolean used[], HashSet<String> set, String cur){
        if(set.contains(cur)) return;
        set.add(cur);