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给定n个有标号的球,标号依次为1,2,…,n。将这n个球放入r个相同的盒子里,不允许有空盒,其不同放置方法的总数记为S(n,r)。例如,S(4,2)=7,这7种不同的放置方法依次为{(1) , (234)} , {(2) , (134)} , {(3) , (124)} , {(4) , (123)} , {(12) , (34)} , {(13) , (24)} , {(14) , (23)}。当n=7,r=4时,S(7,4)= 。
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