Method backtrack

LetterTilePossibilities-1.java:11–20

(String tiles, boolean used[], HashSet<String> set, String cur)

        return set.size() - 1;   
    }
    public void backtrack(String tiles, boolean used[], HashSet<String> set, String cur){
        if(set.contains(cur)) return;
        set.add(cur);
        for(int i=0;i<len;i++){
            if(used[i]) continue;
            used[i] = true;
            backtrack(tiles,used,set,cur + tiles.charAt(i));
            used[i] = false;
        }
    }
}

// recursion with freq