Add a new parameter `b`

--means begin number.

The range of `x`

is : `[ b,11-k )`

```
def combinationSum3(self, k, n,b=1):
if k == 1:
return (b <= n < 10) * [[n]]
return [[x] + r for x in range(b, 11-k)
for r in self.combinationSum3(k-1, n-x, x+1)]
```