Rewrite CSES Flight Routes Check Proof

[izhang05]

I think writing the proof without the unnecessary contradiction makes it a lot clearer and easier to read.

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[chezbgone]

From Discord discussion: Is the proof even necessary? I feel like adding an extra sentence to the first line of the "Main Idea" section would be better. The sentence can be something like "This is because if we want to find a path u -> v (u \to v), we can compose the two paths u -> x and x -> v.

[izhang05]

Yeah, in that case I'm not sure we need the can[u][v] either. It would probably be clearer to say something like "If for every node $x$, there is a path from $1$ to $x$ and a path from $x$ to $1$..."

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. Please address the requested changes and re-request reviews. Thank you for your contribution!

[stale[bot]]

Changes requested have not been made. Free free to create a new PR.