```
class Solution(object):
def combinationSum3(self, k, n):
if k<1 or k> 9 or n<1 or n>45:
return []
path, res, index = [], [], 1
self.dfs(k, n, index, path, res)
return res
def dfs(self, k, n, index, path, res):
if n < 0 or len(path)>k:
return
if n == 0 and len(path)==k:
res.append(path)
for i in range(index, 10):
self.dfs(k, n-i, i+1, path+[i], res)
```