Given an array [a1, a2, ..., aN, b1, b2, ..., bN, c1, c2, ..., cN] convert it to [a1, b1, c1, a2, b2, c2, ..., aN, bN, cN] in-place using constant extra space.
http://www.ardendertat.com/2011/10/18/programming-interview-questions-9-convert-array/