A. Did We Get Everything Covered?time limit per test2 secondsmemory limit per test256 megabytesinputstandard inputoutputstandard outputYou 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.InputThe 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.OutputFor 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).ExampleInput
32 2 4abba2 2 3abb3 3 10aabbccababOutput
YES
NO
aa
NO
ccc
NoteFor 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.
32 2 4abba2 2 3abb3 3 10aabbccabab
YES NO aa NO ccc 
['constructive algorithms', 'dp', 'greedy', 'shortest paths', 'strings', '*1500']