题目网址: http://www.puzzleup.com/2018/
http://www.puzzleup.com/2018/puzzle/?19
答题时限: １２月13日７ＰＭ－比赛结束（约12月19日）
加分时限: １２月13日７ＰＭ－１２月19日６：５９ＰＭ
答对可得基本分１００分。答案可上传５次，每改１次答案从基本分扣２０分。
比赛期间内可随时上传答案，加分时限内答对第ｎ天加（６－ｎ）分
另依题目的难易有额外加分（如有８０％的人这题答错，答对者加８０分）
◆CODE SETS
You will create code sets using A, B, C, D, E, F letters.
‧ A set contains at least two codes.
‧ Each letter is used exactly once by the code set.
‧ Each code contains at least two letters.
‧ When the order of codes in the set changes, it is still considered
the same code set.
‧ When the order of letters in a code changes, the code set is
considered different.
How many different code sets can be created?
If the question were asked for A,B,C,D letters, the answer would be 12.
(AB, CD), (AB, DC), (AC, BD),
(AC, DB), (AD, BC), (AD, CB),
(BA, CD), (BA, DC), (CA, BD),
(CA, DB), (DA, BC), (DA, CB).
你要用A,B,C,D,E,F等字母造出编码组合。
．每个组合至少包含两段编码。
．每个字母都会在编码组合中出现刚好一次。
．每段编码至少包含两个字母。
．组合中的编码次序改变时，视为相同组合。
．编码中的字母次序改变时，视为不同编码。
问能造出多少种不同的编码组合？
如果问题只有A,B,C,D四个字母的话，答案会是12。
(AB, CD), (AB, DC), (AC, BD),
(AC, DB), (AD, BC), (AD, CB),
(BA, CD), (BA, DC), (CA, BD),
(CA, DB), (DA, BC), (DA, CB).