给出一个长度为 n 的数列，$2\leq n\leq 4\ast 10^5$，每个数字 $a_i$ 都在 $[1,20]$ 内。
You are given a list of strings over the alphabet A (for adenine), C (cytosine), G (guanine), and T (thymine), and your task is to find the shortest string (which is typically not listed) that contains all given strings as substrings. If there are several such strings of shortest length, find the smallest in alphabetical/lexicographical order.
Time Limit: 4 second(s)
Memory Limit: 32 MB