书架上的排列问题 We have k books to arrange on the n shelves of a bookcase.The order in which the books appear on a shelf matters,and each shelf can hold all the books.We will assume that as the books are placed on the shelves,they are pushed as far to the left as they will go.Thus,all that matters is the order in which the books appear.When book i is placed on a shelf,it can go between two books already there or to the left or right of all the books on that shelf. na+1a+2)…a+k-)=Πm+i-) -a+》=a+k-y k-1 =(n+k-1)/(n+k-1)-k！ (n-1)1 j=0书架上的排列问题 =(n+k-1)!/((n+k-1)-k)!