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%I #5 Nov 28 2020 09:15:16
%S 1,2,12,13,23,14,34,15,45,16,56,17,67,18,78,19,89,101,102,24,25,35,26,
%T 46,27,57,28,68,29,79,201,103,36,37,47,38,58,39,69,301,104,48,49,59,
%U 401,105,501,106,601,107,701,108,801,109,901,112,113,202,203,302,204,402,205,502,206,602,207,702,208,802,209,902
%N Lexicographically earliest sequence of distinct nonnegative terms such that the last digit d of a(n), for n > 1, is the sum of the two closest digits of d (they are the leftmost digit of a(n+1) and the digit on the left of d).
%C The last two digits of a(n) must be in ascending order (thus no term of the sequence ends in 0).
%e a(2) = 2 and 2 is the sum of 1 + 1 (closest digits to 2);
%e a(3) = 12 and 2 is the sum of 1 + 1 (closest digits to 2);
%e a(4) = 13 and 3 is the sum of 1 + 2 (closest digits to 3);
%e a(5) = 23 and 3 is the sum of 2 + 1 (closest digits to 3); etc.
%Y Cf. A339138 (where the first digit is involved, instead of the last digit)
%K base,nonn
%O 1,2
%A _Eric Angelini_, Nov 25 2020
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