Puzzleup 2011 (06) Chess Players - 西洋棋

題目網址: www.puzzleup.com/2011/puzzle/?224
加分時限: 9/1 7PM - 9/6 7PM

答對可得基本分１００分。答案可上傳５次，每改１次答案從基本分扣２０分。　
比賽期間內可隨時上傳答案，加分時限內答對第ｎ天加（６－ｎ）分　 　　 　　
另依題目的難易有額外加分（如有８０％的人這題答錯，答對者加８０分）　　

◆Chess Players

25 players are attending a chess tournament. In every possible combination of
five players, at least one of the players has played with the other four
players before.

What is the minimum possible number of players in the tournament that has
played with all the other 24 players before?

25個棋手參加一場西洋棋大賽。
任意選擇五個棋手，總會至少有一人在之前和其他四人都對戰過。

那麼這些人中最少有幾個人和除自己之外的24人都對戰過？

--
又是圖論題...= =

--
'You've sort of made up for it tonight,' said Harry. 'Getting the
sword. Finishing the Horcrux. Saving my life.'
'That makes me sound a lot cooler then I was,' Ron mumbled.
'Stuff like that always sounds cooler then it really was,' said
Harry. 'I've been trying to tell you that for years.'
-- Harry Potter and the Deathly Hollows, P.308

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