"""This problem was asked by Coinbase.

Write a function that takes in a number, string, list, or dictionary and 
returns its JSON encoding. It should also handle nulls.

For example, given the following input:

[None, 123, ["a", "b"], {"c":"d"}]
You should return the following, as a string:

'[null, 123, ["a", "b"], {"c": "d"}]'
"""