Data Structures, Algorithms, & Applications in C++
Chapter 18, Exercise 15

initial       [2 3 6 8 11 15] [0 7 12] [1 4 13] [5 9 10 14]
merge to b    [0 2 3 6 7 8  11 12  15] [1 4 5 9 10 13 14]
merge to a    [0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15]