merge sort, top-down version:

7   5a  8   6   4   5b  3   9   1
7   5a  8   6   4 | 5b  3   9   1
7   5a  8 | 6   4 | 5b  3 | 9   1
7   5a| 8 | 6 | 4 | 5b| 3 | 9 | 1
7 | 5a| 8 | 6 | 4 | 5b| 3 | 9 | 1
5a  7 | 8 | 6 | 4 | 5b| 3 | 9 | 1
5a  7   8 | 4   6 | 3   5b| 1   9
4   5a  6   7   8 | 1   3   5b  9
1   3   4   5a  5b  6   7   8   9


merge sort, bottom-up version:

7   5a  8   6   4   5b  3   9   1
7 | 5a| 8 | 6 | 4 | 5b| 3 | 9 | 1
5a  7 | 6   8 | 4   5b| 3   9 | 1
5a  6   7   8 | 3   4   5b  9 | 1
3   4   5a  5b  6   7   8   9 | 1
1   3   4   5a  5b  6   7   8   9