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Kotlin-based LeetCode algorithm problem solutions, regularly updated
216\. Combination Sum III
Medium
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.
**Example 1:**
**Input:** k = 3, n = 7
**Output:** [[1,2,4]]
**Explanation:** 1 + 2 + 4 = 7 There are no other valid combinations.
**Example 2:**
**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.
**Example 3:**
**Input:** k = 4, n = 1
**Output:** []
**Explanation:** There are no valid combinations. Using 4 different numbers in the range [1,9], the smallest sum we can get is 1+2+3+4 = 10 and since 10 > 1, there are no valid combination.
**Constraints:**
* `2 <= k <= 9`
* `1 <= n <= 60`