Date: Nov 2019
Location: NYC

Was asked a small variation of the Merge K Sorted List question -

Given a list of files, each file containing any number of sorted strings, merge them into a single file.
You can use the following methods as helpers if needed -

File.readLine() -> returns the next line in the file
File.hasNext() -> boolean if this has more lines
File.writeLine(String) -> write a line to the file

The signature of the method is public File merge(List<File> files) { ... }

At the end of the code implementation, was asked to write the different test cases that the code should be tested against as well as implement a few tests using Junit and any Mock framework of choice.
The tests cases I suggested were -

1. Empty input list
2. Null input list
3. happy case - multiple files, each file having multiple alphabetical strings in them
4. one or more file having non-alphabet chars (e.g. special chars)
5. multiple files, but one or more of them is empty / null
6. list having just 1 file