ab12

swap b, 1 (1, 2 in 4)

a1b2

-

abc123

swap b, 1 (1, 3 in 6)

a1cb23

swap c, b (2, 3 in 6)

a1bc23

swap c, 2 (3, 4 in 6)

a1b2c3

--

abcd1234

swap b, 1 (1, 4 in 8)

a1cdb234

swap c, b (2, 4 in 8)

a1bdc234

swap d, 2 (3, 5 in 8)

a1b2cd34

swap d, 3 (5, 6 in 8)

a1b2c3d4

---

abcde12345

swap b, 1 (1, 5 in 10)

a1cdeb2345

swap c, b (2, 5 in 10)

a1bdec2345

swap d, 2 (3, 6 in 10)

a1b2ecd345

swap e, c (4, 5 in 10)

a1b2ced345

swap e, 3 (5, 7 in 10)

a1b2c3de45

swap e, 4 (7, 8 in 10)

a1b2c3d4e5

----

abcdef123456

swap b, 1 (1, 6 in 12)

a1cdefb23456

swap c, b (2, 6 in 12)

a1bdefc23456

swap d, 2 (3, 7 in 12)

a1b2efcd3456

swap e, c (4, 6 in 12)

a1b2cfed3456

swap f, 3 (5, 8 in 12)

a1b2c3edf456

swap e, d (6, 7 in 12)

a1b2c3def456

swap e, 4 (7, 9 in 12)

a1b2c3d4fe56

swap f, e (8, 9 in 12)

a1b2c3d4ef56

swap f, 5 (9, 10 in 12)

a1b2c3d4e5f6

-----

swap 1, length/2
swap 2, length/2
swap 3, length/2+1
...
swap length-3, length-2

2
33      4
44     56
5565   78
66768799A