CF1924A.Did We Get Everything Covered?

You are given two integers $n$ and $k$ along with a string $s$.

Your task is to check whether all possible strings of length $n$ that can be formed using the first $k$ lowercase English alphabets occur as a subsequence of $s$. If the answer is NO, you also need to print a string of length $n$ that can be formed using the first $k$ lowercase English alphabets which does not occur as a subsequence of $s$.

If there are multiple answers, you may print any of them.

Note: A string $a$ is called a subsequence of another string $b$ if $a$ can be obtained by deleting some (possibly zero) characters from $b$ without changing the order of the remaining characters.

给你两个整数 $n$ 和 $k$，以及一个字符串 $s$。

你的任务是判断：所有由前 $k$ 个小写英文字母构成的长度为 $n$ 的字符串，是否都作为 $s$ 的子序列出现。如果答案为 NO，你还需要输出一个由前 $k$ 个小写英文字母构成的、长度为 $n$ 且不是 $s$ 的子序列的字符串。

如果有多个满足条件的答案，你可以输出其中任意一个。

注意：若字符串 $a$ 可通过从字符串 $b$ 中删除若干（可能为零）个字符、且不改变剩余字符的相对顺序而得到，则称 $a$ 是 $b$ 的子序列。

The first line of input contains a single integer $t \, (1 \le t \le 10^5)$, the number of test cases.

The first line of each test case contains $3$ integers $n \, (1 \le n \le 26), \: k \, (1 \le k \le 26), \: m \, (1 \le m \le 1000)$, where $n$ and $k$ are the same as described in the input and $m$ is the length of the string $s$.

The second line of each test case contains a single string $s$ of length $m$, comprising only of the first $k$ lowercase English alphabets.

It is guaranteed that the sum of $m$ and the sum of $n$ over all test cases does not exceed $10^6$.

输入的第一行包含一个整数 $t \, (1 \le t \le 10^5)$，表示测试用例的数量。

每个测试用例的第一行包含三个整数 $n \, (1 \le n \le 26), \: k \, (1 \le k \le 26), \: m \, (1 \le m \le 1000)$，其中 $n$ 和 $k$ 的含义与题面描述一致，$m$ 是字符串 $s$ 的长度。

每个测试用例的第二行包含一个长度为 $m$ 的字符串 $s$，该字符串仅由前 $k$ 个小写英文字母组成。

保证所有测试用例中 $m$ 的总和以及 $n$ 的总和均不超过 $10^6$。

For each test case, print YES if all possible strings of length $n$ that can be formed using the first $k$ lowercase English alphabets occur as a subsequence of $s$, else print NO.

If your answer is NO, print a string of length $n$ that can be formed using the first $k$ lowercase English alphabets which does not occur as a subsequence of $s$ in the next line.

You may print each letter of YES or NO in any case (for example, YES, yES, YeS will all be recognized as a positive answer).

对于每个测试用例，如果所有由前 $k$ 个小写英文字母构成的长度为 $n$ 的字符串均作为 $s$ 的子序列出现，则输出 YES；否则输出 NO。

若答案为 NO，请在下一行输出一个由前 $k$ 个小写英文字母构成、且不作为 $s$ 的子序列出现的长度为 $n$ 的字符串。

你可以以任意大小写形式输出 YES 或 NO 中的每个字母（例如，YES、yES、YeS 均被视为正确答案）。

For the first test case, all possible strings (aa, ab, ba, bb) of length $2$ that can be formed using the first $2$ English alphabets occur as a subsequence of abba.

For the second test case, the string aa is not a subsequence of abb.

对于第一个测试用例，所有由前 $2$ 个英文字母构成的长度为 $2$ 的可能字符串（即 aa、ab、ba、bb）均作为子序列出现在字符串 abba 中。

对于第二个测试用例，字符串 aa 不是字符串 abb 的子序列。