In the process of writing some unit tests, I needed to compare the output string with the expected output (. Except the strings are XML snippets, and I didn't want the tests to break if I changed the formatting. So I need to write up a simple "string compare, whitespace insensitive" function (e.g " f o o " == "foo"). Very handy, but not built into any programming language that I can recall.
Writing this function is a good simple programming, weed-out question for interviewing. It works for any programming language, too. The higher level languages will have a few ways to do it. If the interviewee can't come up with something, well, it will be a short interview.