Problem 2031. Half-Swap

Given a vector with an even number of elements, rearrange it so that the elements in its first half are switched with those in the second half.

For example, given a vector:

 Vec = [23  24  6  15  15  20  22  23  9  21]

will result in

 Vec = [20  22  23  9  21  23  24  6  15  15]