Alien Dictionary

There is a new alien language that uses the English lowercase letters, but the **order** among the letters is unknown to you. You are given `words`, a list of strings from the alien language's dictionary. The strings in `words` are sorted **lexicographically** by the rules of this new language. Return a string of the unique letters in the new alien language sorted in **lexicographically increasing order** by the new language's rules. If there is no valid ordering, return `""`. If there are multiple valid orderings, return **any** of them. Implement `function alienOrder(words)`. Note: a string `s` is lexicographically smaller than a string `t` if at the first letter where they differ, the letter in `s` comes before the corresponding letter in `t`. If the first `min(s.length, t.length)` letters are the same, then `s` is smaller iff `s.length < t.length`.