You are given a list of words sorted in lexicographic order according to an unknown ("alien") alphabet. ### Task Return **one valid ordering of the unique characters** in this alien alphabet that is consistent with the given sorted word list. If **no valid ordering exists**, return an empty string. ### Details - The alien alphabet contains exactly the set of distinct characters that appear in the input words. - The input list `words` is already sorted according to the alien alphabet. - If multiple valid character orders exist, you may return any one of them. ### Input - `words`: an array of strings, length `n`. ### Output - A string containing all distinct characters exactly once in a valid order, or `""` if impossible. ### Constraints (typical interview bounds) - `1 <= n <= 10^4` - `1 <= len(words[i]) <= 100` - Characters are lowercase English letters (you can assume an upper bound of 26 unique characters). ### Notes / Edge cases - If `word1` is a strict prefix of `word2`, then `word1` must appear before `word2` in the sorted list (otherwise the ordering is invalid). ### Example Input: `words = ["wrt", "wrf", "er", "ett", "rftt"]` Possible output: `"wertf"`