The hardest one I ever had was to make word ladder perform in linear time. The only time I saw anything similar was in an old ACM-ICPC competition problem set.

The hardest one I ever had was to make word ladder perform in linear time. The only time I saw anything similar was in an old ACM-ICPC competition problem set. It's not just that they ask coding questions, it's that they demand you wring ever optimization possible out of them even when you might not see any other way to do it.