A043225 Numbers k such that 1 and 2 occur juxtaposed in the base-10 representation of k but not of k-1.

12, 21, 112, 120, 210, 221, 312, 321, 412, 421, 512, 521, 612, 621, 712, 721, 812, 821, 912, 921, 1012, 1021, 1112, 1120, 1200, 1312, 1321, 1412, 1421, 1512, 1521, 1612, 1621, 1712, 1721, 1812, 1821, 1912, 1921, 2012, 2021