input: Write a python function to recursively list all files in a directory.
  assert set(walk("/path/to/directory")) == set(["/path/to/directory/file1.txt", "/path/to/directory/file2.txt", "/path/to/directory/subdir/file3.txt"])
  assert set(walk("/another/path")) == set(["/another/path/fileA.py", "/another/path/fileB.js", "/another/path/deeper/fileC.cpp"])
output: import os
  
  def walk(directory):
      files = []
      for entry in os.scandir(directory):
          if entry.is_file():
              files.append(entry.path)
          elif entry.is_dir():
              files.extend(walk(entry.path))
      return files