Added LFU Cache

[ruppysuppy]

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[TravisBuddy]

Travis tests have failed

Hey @ruppysuppy,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: b7131530-b5e9-11ea-8cfa-25e523bed74d

[cclauss]

The comment said that None was returned but the failing test demonstrates that is not the case.

[ruppysuppy]

@cclauss, when None is returned in python interpreter or doctests, its not shown. For example try this code in the interpreter:

>>> def f():
	return None

>>> f()
>>> 

Even though f returns None, running f() won't display None in the interpreter or doctests (unless print(f()) is explicitly called)

[l3str4nge]

Please change ''' to """ in docstrings

[cclauss]

autoblack will do this automatically so let's not slow the review process on this.

[l3str4nge]

Could LFUCache take optional cache parameter? Would be easier to test.

 def __init__(self, capacity: int, cache: dict = None):
        self.list = DoubleLinkedList()
        self.capacity = capacity
        self.num_keys = 0
        self.hits = 0
        self.miss = 0
        self.cache = cache or {}

[cclauss]

That seems like a big change.

[cclauss]

LGTM

Looks good.