title: How to swap to elements in Seq
body: <p>I have an exercise where I have to swap elements on even and odd positions.
  For example, from <code>Seq(1,2,3,4,5)</code> I have to get <code>Seq(2,1,4,3,5)</code>.</p>
  <p>I wanted to use sliding and then swap two elements in the slid Seq, but sliding will take something like this: <code>(1,2) (2,3) (3,4) (4,5)</code>, won't it? Is there any function to take only unique pairs?</p>