Jigsaw Puzzle Solving

Imagine a usual jigsaw puzzle. Let its picture be a completely white rectangle with the width of W pixels and the height of H pixels. The puzzle was splitted into K pieces, then they've got mixed up. Now you want to restore it again. Ideally, you'd like merge these pieces in such a way that they'd form a rectangle W pixels wide and H pixels tall, again. However, it can be very hard. Therefore, e