Welcome back,

来源: 于 2010-11-11 16:59:18 [旧帖] [给我悄悄话] 本文已被阅读：次

The answer is n,

We use Balance method:

M1M2.....MnW1W2....Wn    M1W1M2W2M3W3....MnWn   The first is 1,second is 2n-1,average is n,The number of each type both are 2*2n!/(n!*n!)......every pemutation with index n+i,can find dual permutation with index n-i,the number are the same.So the answer is n.