Updated has_loop lookup from O(n) to O(1)

[1227haran]

Describe your change:

visited variable is declared as a list, hence node in visited takes a TC of O(n). This issue is fixed by declaring visited as a set reducing the lookup TC to O(1).

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• 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.
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[1227haran]

All the tests are passing, please approve my PR

[tianyizheng02]

Thanks for your contribution!