Alien Dictionary

There is a new alien language that uses the English alphabet, but the order among the letters is unknown. You are given a list of words from the alien dictionary's lexicon, sorted lexicographically by the rules of this new language.

Derive one valid ordering of letters in this language, and return it as a string. If there's no valid ordering, return "".

Example 1:

Input: words = ["wrt","wrf","er","ett","rftt"]
Output: "wertf"

Example 2:

Input: words = ["z","x"]
Output: "zx"

Example 3:

Input: words = ["z","x","z"]
Output: ""
Explanation: "z" would need to come both before and after "x" — contradiction.

Constraints:

• 1 ≤ words.length ≤ 100
• 1 ≤ words[i].length ≤ 100
• words[i] consists of only lowercase English letters