题目网址: http://www.puzzleup.com/2014/
http://www.puzzleup.com/2014/puzzle/?251
答题时限: 10月２日７ＰＭ－比赛结束（约12月17日）
加分时限: 10月２日７ＰＭ－10月８日６：５９ＰＭ
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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）　　
◆Five-Letter Code
You will produce five-letter codes using the letters A, B, C, D, and E.
You are not allowed to produce a code that can be constructed by
interchanging any two adjacent letters of an existing code.
What is the maximum number of different codes that can be produced according
to this rule?
If the problem was asked for three-letter codes using the letters A, B, and C,
then the answer would be 18.
AAA, AAB, AAC, ABB, ABC, ACC,
BAA, BBA, BBB, BBC, BCA, BCC,
CAA, CAB, CBB, CCA, CCB, CCC.
你要使用 A B C D E 五个字母组成长度为五的编码，
但你不能使用已有编码中交换相邻两个字母所形成的编码。
试问最多可以组成多少个编码？
若题目改问 A B C 三个字母组成长度三的编码, 则答案为 18：
AAA, AAB, AAC, ABB, ABC, ACC,
BAA, BBA, BBB, BBC, BCA, BCC,
CAA, CAB, CBB, CCA, CCB, CCC.