Alien Dictionary

An alien language uses lowercase English letters, but in an unknown alphabetical order. You are given a list words that is claimed to be sorted in that alien order. Words are compared the usual dictionary way: look at the first position where they differ, and the word with the earlier letter comes first; if one word is a prefix of the other, the shorter word comes first.

Return a string containing every distinct letter that appears in words, each exactly once, in an order under which words is sorted. Letters must not appear if they are not in words.

• If several orders are valid, return any one of them.
• If no order can make words sorted (the comparisons contradict each other, or a word appears before a shorter word that is its own prefix), return "".