Alien Dictionary

There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of non-empty words from the dictionary, where words are sorted lexicographically by the rules of this new language. Derive the order of letters in this language.

Example 1:

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

Example 2:

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