To add "Combination Sum-III" problem of leetcode no-216

[Amit-yadav099]

hello @JenilGajjar20 i want to contribute a code of combination sum -III in your list which is leetcode -216 as a newbie in contribution, kindly consider this and assign this work to me under hacktoberfest2024.

problem statement:- Find all valid combinations of k numbers that sum up to n such that the following conditions are true: Only numbers 1 through 9 are used. Each number is used at most once. 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.

sample testcase: Input: k = 3, n = 9 Output: [[1,2,6],[1,3,5],[2,3,4]] Explanation: 1 + 2 + 6 = 9 1 + 3 + 5 = 9 2 + 3 + 4 = 9 There are no other valid combinations.