I think it's just an interesting approach to solving particular limited problems. If I needed to solve this I'd end up either using set arithmetic or sorting the list, both of which use more memory and time. Maybe down low in some compiler loop or JVM loop this could be the difference between a sluggish application and a snappy one

That's not my point. My point is that the exact same code from the original article could be done in a single, traditional for-loop, instead of two for-each loops.