Good morning! Here's your coding interview problem for today.

This problem was asked by Microsoft.

Implement a URL shortener with the following methods:

shorten(url), which shortens the url into a six-character alphanumeric string, such as zLg6wl.
restore(short), which expands the shortened string into the original url. If no such shortened string exists, return null.
Hint: What if we enter the same URL twice?