Fill in the get_num_sub_arrays function in the empty.py file. Non optimal soltuion can be found in the non_optimal.py file.

The function should return the number of subarrays in the input list arr that have a product of its elements less than or equal to k.

For example, for the input list arr = [1, 2, 3, 4] and k = 6, the function should return 7 because the subarrays that have a product of its elements less than or equal to k are: [1], [2], [3], [4], [1, 2], [2, 3], [1, 2, 3]

One of the test makes sure the solution works optimally when the input list is large and the product limit is small.