prep的一道数学

tartan

A certain stock exchange designates each stock with a one-, two-, or three-letter code, where each letter is sellected from the 26 letters of the alphabet. If the letters may be repeated and if the same letters used in a different order consitute a different code, how many different stocks is it possible to uniquely designate with these codes?
答案是18，278
可我怎么也算不到那么大啊~还是我想的太简单了~难道不是 A(26,1)+A(26,2)+A(26,3)??
我脑子可能短路了~还请大家帮忙啊
谢谢啦

qq6501215

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xiaotangyu

同问！