eg:输入:k=3,n=9
输出: [[1,2,6],[1,3,5],[2,3,4]]
输入:k=2,n=5
输出:[[1,4][2,3]]
------------------------------------------------------------------------------------
from itertools import combinations
def num(k,n):
nums=[1,2,3,4,5,6,7,8,9]
res=[]
for i in range(len(nums)):
res +=list(combinations(nums,i))
res=[x for x in res if len(x) == k]
a=[]
for j in res:
if sum(j) ==n :
a.append(list(j))
return a