Add an example of an adjacency list

[anukem]

Issue Description

Adding an example of an adjacency list to the 99 problems set.

Fixes # (issue)

Changes Made

Please describe the changes that you made.

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• [ ] PR is descriptively titled and links the original issue above
• [ ] Before/after screenshots (if this is a layout change)
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[anukem]

@patricoferris Any luck here?

[anukem]

Closing this PR out since it doesn't seem to be getting much engagement.