w****a
发帖数: 155 | 1 有n个盒子，有m个球，n >m , 每个球放入不同盒子的概率是相等的，都是1/n.
请问最后这m个球放入x个盒子的概率是多少，x < m. | e****d
发帖数: 333 | 2 answer=(n!/n^m/(n-x)!)*S(m,x)
where S is the Stirling number of the second kind.
The recursive definition is:
S(m,x)=x*S(m-1,x)+S(m-1,x-1)
with initial conditions:
S(r,1)=S(r,r)=1.
ie.
n=4;m=4;x=2;
answer=0.328.
I've posted the same answer at Quant, jobHunting,statistics forum for you.
Personally, i won't expect such a problem to appear at any interview. |
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