Return all non-negative integers of length N such that the
absolute difference between every two consecutive digits is K.
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.
Example 1:
Input: N = 3, K = 7 Output: [181,292,707,818,929] Explanation: Note that 070 is not a valid number, because it has leading zeroes.
Example 2:
Input: N = 2, K = 1 Output: [10,12,21,23,32,34,43,45,54,56,65,67,76,78,87,89,98]