It's not a work for hire and there is no copyright assignment so copyright will remain with the candidate. However some whiteboard scribbles are not the same a

It's not a work for hire and there is no copyright assignment so copyright will remain with the candidate. However some whiteboard scribbles are not the same as working code, so it is likely easy to make an implementation that does not infringe in most cases. Not that it really matters because it's highly unlikely that anyone is going to sue over it.