Given an array of strings words, return the smallest string that contains each string in words as a substring. If there are multiple valid strings of the smallest length, return any of them. You may assume that no string in words is a substring of another string in words. Example 1: Input: words = ["alex","loves","leetcode"] Output: "alexlovesleetcode" Explanation: All permutations of "alex","loves","leetcode" would also be accepted. Example 2: Input: words = ["catg","ctaagt","gcta","ttca","atgcatc"] Output: "gctaagttcatgcatc" Constraints: 1 <= words.length <= 12 1 <= words[i].length <= 20 words[i] consists of lowercase English letters. All the strings of words are unique.