Find all valid combinations of k numbers that sum up to n such that the following conditions are true:
1 through 9 are used.Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.
2 <= k <= 91 <= n <= 60k = 3, n = 7[[1,2,4]]k = 3, n = 9[[1,2,6],[1,3,5],[2,3,4]]k = 4, n = 1[]