There is a new alien language that uses the English alphabet. However, the order among the letters is unknown to you.
You are given a list of strings words from the alien language's dictionary, where the strings in words are sorted lexicographically by the rules of this new language.
Return a string of the unique letters in the new alien language sorted in lexicographically increasing order by the new language's rules. If there is no solution, return " ". If there are multiple solutions, return any of them.
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: The order is invalid, so return " ".
Constraints:
1 <= words.length <= 1001 <= words[i].length <= 100words[i] consists of only lowercase English letters.