Efficient Cartesian Product algorithm

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The complexity of cartesian product is O(n2), there is no shortcut.

In specific cases, the order you iterate your two axis is important. For example, if your code is visiting every slot in an array — or every pixel in an in image — then you should try to visit the slots in natural order. An image is typically laid out in ‘scanlines’, so the pixels on any Y are adjacent. Therefore, you should iterate over the Y on the outer loop and the X on the inner.

Whether you need the cartesian product or wherther is a more efficient algorithm depends on the problem you are solving.

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