Alien Dictionary

Write `alien_order(words)` returning a letter order consistent with the given sorted word list, or `""` if none exists. Any valid order is accepted. ["wrt","wrf","er","ett","rftt"] -> "wertf" Compare adjacent words, find the first differing character, and that is one edge. Two traps: a word followed by its own proper prefix (`["abc","ab"]`) is invalid input and must return `""`; and every letter appearing anywhere must be in the output, even letters that produce no edges.