Improve Project Euler problem 014 solution 2

[MaximSmolskiy]

Describe your change:

Improve Project Euler problem 014 solution 2 - the top 1 slowest solution on Travis CI logs (under slowest 10 durations: 14.20s call scripts/validate_solutions.py::test_project_euler[problem_014/sol2.py]):

• Improve solution (locally 10+ times - from 15+ seconds to ~1.5 seconds)

• Uncomment code that has been commented due to slow execution affecting Travis (now it should be quite fast execution and not affect Travis)

• 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}.

[IdoErel]

Nice work overall. You replaced the loop with recursion, possibly making the code faster. Also, having the sequence lengths as a global dictionary would save a lot of time computing.

Maybe note such changes in the commit message next time (E.g. loop to recursion).

Sincerely,
IdoErel

[IdoErel]

Unnecessary variable assignment.
This can be shortened to one line.

[MaximSmolskiy]

But then we should return COLLATZ_SEQUENCE_LENGTHS[n] instead of sequence_length. I don't know Python perfectly - whether there will be an extra search in the dictionary or not?

[IdoErel]

No, you're right. I missed line 43.
Disregard my previous comment.

I originally meant, that assigning the result to sequence_length - and immediately after assigning it again to COLLATZ_SEQUENCE_LENGTHS, without any further usage is unnecessary. (of sequence_length).

However, you're returning sequence_length as the result. Saving another lookup - in short, keep it this way. I was wrong.

Sorry for my ignorance.

Sincerely,
IdoErel