Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

Credits:

class Solution:
    def combinationSum3(self, k, n):
        """
        :type k: int
        :type n: int
        :rtype: List[List[int]]
        """
        res = []
        def gen(l, k, n, cur):
            if k == len(l) and
 
 n == 0:
                res.append(l[:])
                return
            for i in range(cur, 10):
                l.append(i)
                gen(l, k, n - i, i + 1)
                l.pop()
                
        gen([], k, n, 1)
        return res

216. Combination Sum III 組合總數三