```
def combination_sum3(k, n)
def comb_sum(k, n, start)
return (n == 0? [[]] : []) unless k > 0
return [] unless start <= 9
comb_sum(k-1, n-start, start+1).map{|x|[start]+x} + comb_sum(k, n, start+1)
end
comb_sum(k, n, 1)
end
```