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Stage 2

matches = [["Alice", "Bob"], ["Charlie", "David"], ["Alice", "Charlie"], ["Bob", "David"]]

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Stage 3

matches = [["Alice", "Bob"], ["Yuki", "Zoe"]]

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Stage 4: hostile inputs

The match list arrives as JSON over an API from a third party. You do not control what they send.

[]                                        # no matches at all
[["Alice", "Alice"]]                      # a player against themselves
[["Alice", "Bob"], ["Alice", "Bob"]]      # the same match twice
[["Alice"]] # arity is not guaranteed

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Stage 5: a full season

The rights holders come back with the real fixture list.

matches = [[f"player{i}", f"player{i+1}"] for i in range(50_000)]

Run it.

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Stage 8: the rights holders return

Reason about these out loud. No implementation needed.

1. Three teams. They now want players split across Team A, B and C, same rule: no player faces a teammate. How does your approach change?

2. Best effort. The fixture list has no valid two-team split, but they refuse to drop matches. Instead they want the assignment that honours as many of the requested matches as possible. What is this problem?