// Alien Dictionary — HARD

// Category: graph

You are given a list of words from an alien language dictionary, where the words are sorted lexicographically by the rules of that language.

Derive the order of characters in the alien alphabet. Return a string of the unique characters in the correct order. If the order is invalid (circular dependency), return an empty string `""`.

If multiple valid orderings exist, return any of them.

Example: words = ["wrt","wrf","er","ett","rftt"]

Output: "wertf"