Let dp[a][b] be the number of numbers with length a that end in the digit b (a ranges from 1 to n, and b ranges from 0 to 9). Then dp[1][b] = 1 for all b. For a > 1 and a fixed value of b, initialize it to zero. Then, loop over all possible values of the second to last digit b', and if |b — b'| >= 4, add dp[a-1][b'] to dp[a][b].

I am not able to understand the problem, can you please post the problem link or elaborate here. Thanks.

I do not have the problem link.So I am elaborating the question. You have to answer the possible ways of n digit number such that the next digit has absolute difference of at least 4 than the previous digit.

An implementation based on mkirsche's comment with O(nm) where m is number of 2-digit numbers with digits having absolute difference >= 4, which is 42

Code