Hacking

A coach of one of the soccer World Cup finalist teams — call him Hugo Hacker — wants to uncover secret information about an opposing team before the match. The opposing coach runs a website with public information about his team, and Hugo suspects that secret information is also stored on the computer that hosts the website.

The website has a form that lets you search for a keyword and returns a chunk of a text file containing that keyword. Hugo has discovered that by entering a word that does not appear in the publicly available documents, he can exploit a bug in the search and gain access to other files on the computer. He already knows the publicly available documents. However, the search box limits the maximum length of a word and the set of characters that may be entered. Can you find a word that can be typed into the search box and that does not occur as a substring of the documents?

The first line contains the number of test cases that follow. Each test case consists of two lines.

The first line has three integers $n$, $m$, and $k$ ($1 \le n \le 10000$, $1 \le m \le 100$, $1 \le k \le 26$): $n$ is the length of the publicly available documents, $m$ is the maximum word length the search box accepts, and $k$ means the search box accepts only the first $k$ letters of the alphabet. The second line contains the publicly available documents as a string of $n$ lower-case letters.

For each test case, print on its own line a word that uses only the first $k$ letters of the alphabet and does not occur as a substring of the given documents. To make the answer unique, print the shortest such word; if several shortest words exist, print the lexicographically smallest of them. It is guaranteed that some word of length at most $m$ over the first $k$ letters is not a substring, so this shortest word always has length at most $m$.