Codeforces Round #445 D. Restoration of string【字符串】
A substring of some string is called the most frequent, if the number of its occurrences is not less than number of occurrences of any other substring.
You are given a set of strings. A string (not necessarily from this set) is called good if all elements of the set are the most frequent substrings of this string. Restore the non-empty good string with minimum length. If several such strings exist, restore lexicographically minimum string. If there are no good strings, print "NO" (without quotes).
A substring of a string is a contiguous subsequence of letters in the string. For example, "ab", "c", "abc" are substrings of string "abc", while "ac" is not a substring of that string.
The number of occurrences of a substring in a string is the number of starting positions in the string where the substring occurs. These occurrences could overlap.
String a is lexicographically smaller than string b, if a is a prefix of b, or a has a smaller letter at the first position where a and b differ.
The first line contains integer n (1 ≤ n ≤ 105) — the number of strings in the set.
Each of the next n lines contains a non-empty string consisting of lowercase English letters. It is guaranteed that the strings are distinct.
The total length of the strings doesn't exceed 105.
Print the non-empty good string with minimum length. If several good strings exist, print lexicographically minimum among them. Print "NO" (without quotes) if there are no good strings.
4
ai
lru
cf
cfmailru
3
kek
preceq
cheburek
NO
One can show that in the first sample only two good strings with minimum length exist: "cfmailru" and "mailrucf". The first string is lexicographically minimum.
【题意】：给出很多字符串，要求构造一个字符串，使得所有给出的字符串在这个串当中都是出现次数最多子串。输出长度最短的答案。如果有多个答案，输出字典序最小的。
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