Return all non-negative integers of length
N such that the absolute difference between every two consecutive digits is
Note that every number in the answer must not have leading zeros except for the number
0 itself. For example,
01 has one leading zero and is invalid, but
0 is valid.
You may return the answer in any order.
Input: N = 3, K = 7
Input: N = 2, K = 1
1 <= N <= 9
0 <= K <= 9