In-place interleaving of the two halves of a string

Question

Given a string of even size, say:

abcdef123456

How would I interleave the two halves, such that the same string would become

Answer 1

The solution is here J. Ellis and M. Markov. In-situ, stable merging by way of perfect shuffle. The Computer Journal. 43(1):40-53, (2000).

Also see the various discussions here:

1. https://cs.stackexchange.com/questions/332/in-place-algorithm-for-interleaving-an-array/400#400
2. https://cstheory.stackexchange.com/questions/13943/linear-time-in-place-riffle-shuffle-algorithm.