Problem 4: Write a Bash script that removes all zero length ordinary files in the directory (including those in the sub-directories at all levels) passed as an optional argument. If you do not specify the directory argument, the script uses the current working directory as the default argument. This problem is for practicing bash programming skills. Though there is an easier way to achieve the goal with the find command, the find command is not allowed to appear in your bash script.

Make sure not to use "find" command to find the zero KB file.