A039989 k th digit of a(n) is the number of different digits within 3 of k (not including k).

11, 111, 122, 212, 221, 1111, 1222, 2122, 2212, 2221, 11111, 12221, 21222, 22122, 22212, 111111, 122212, 122221, 133322, 212221, 221222, 222122, 223331, 1111111, 1222122, 1222212, 1222221, 1333222, 2122212, 2122221, 2212221, 2221222