Alien dictionary

You are given a list of words sorted lexicographically according to the rules of an unknown alien language that uses lowercase English letters. Derive any valid ordering of the alphabet consistent with the input. The order between two letters is implied by the first position at which two adjacent words differ. Return a string of the letters in a valid order; if no valid order exists (a contradiction or the prefix rule is violated, e.g. ['abc','ab']), return the empty string. If multiple orders are valid, return any one of them.