```
class Solution:
# @return a list of lists of integers
def combine(self, n, k):
if n<k:
return [[]]
elif k==1:
tot=[]
x=[0]
for i in range(1,n+1):
x[0]=i
tot.append([x[0]])
return tot
elif n==k:
tot=[]
x=[]
for i in range(1,n+1):
x.append(i)
tot.append(x)
return tot
else:
tot=[]
for x in self.combine(n-1,k-1):
y=x+[n]
tot.append(y)
return tot+self.combine(n-1,k)
```