Puzzleup 2014 (4) Four Letters - 拼圖

題目網址: http://www.puzzleup.com/2014/
http://www.puzzleup.com/2014/puzzle/?245
答題時限: ８月21日７ＰＭ－比賽結束（約12月17日）
加分時限: ８月21日７ＰＭ－８月27日６：５９ＰＭ

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）　　

◆Four Letters

You are asked to create a text using only the letters A, B, C, and D. If all
possible four-letter strings (from AAAA to DDDD) are required to be included
within the text, what should be the minimum length of this text?

If the problem was asked for the letters A, and B, then the answer would be 5.
Sample text:AABBA

你被要求只以 A, B, C, D 四個字母構成一串文字。

若所有的四字母組合都要出現 (從 AAAA 到 DDDD)，試問最小字串長度是多少？

若題目改問兩個字母 A, B, 則答案為 5。可能答案：AABBA。

[譯註: 於是看起來是指所有 AAAA AAAB AAAC ... 到 DDDD 一共 256 種組合都要有]

--
'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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