2010年Puzzleup数学竞赛−第10题: Generating 4-Letter Codes

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Generating 4-Letter Codes

    You will create a list of 4-letter codes using the 26 letters in theEnglish alphabet. The list should not include any code that can beobtained by reversing the order of 2, 3 or 4 adjoining letters ofanother code in the list.

    For example, if the list includes ABGG, then it can't include codes BAGG, AGBG, GBAG, AGGB and GGBA.

    At most how many codes can be in this list?