排列、组合

import itertools
mylist=list(itertools.permutations(['A','B','C','D'],4))#全排列
print(mylist)
print(len(mylist))
#itertools.permutations([1,2,3,4],2)排列，4个数的列表，取出3个数，不同的顺序算，
#5!=1*2*3*4*5
#0!=1
#M个选N个   M！/(M-N)！
#4  1    4!/3!=4
#4 2  4!/2！=3*4=12
#4 3  4！/1！=24

import itertools
mylist=list(itertools.combinations(['A','B','C','D'],3))
print(mylist)
print(len(mylist))
#['A','B','C','D'],3) 4个取3个，无序
#M!/N!(M-N)!