Add kotlin code for the chapter of graph

[curtishd]

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[krahets]

Please set the PR's title as "Add kotlin code for the chapter of graph". I've modified the title of this PR

[curtishd]

Hmm, Some imperfections are not easy to spot.

[krahets]

Hmm, Some imperfections are not easy to spot.

Indeed. But for code formatting, we can leave the job to the code formatter.

[krahets]

Great work! Thanks!

[curtishd]

My pleasure