fact(n) = n! = 1 * 2 * 3 * ... * (n-1) * n = (n-1)! * n = fact(n-1) * n最后返回值？？？？
fact(n) = n! = 1 * 2 * 3 * ... * (n-1) * n = (n-1)! * n = fact(n-1) * n
由此可知：fact(n)=fact(n-1)*n
def fact(n):
... if n==1:
... return 1
... return fact(n)
return fact(n)为什么不正确？
