171. Combination Sum III
MediumBacktracking
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.
Examples
Input: 3 7
Output: [[1,2,4]]
Explanation: Only 1+2+4 = 7 uses three distinct digits from 1..9.
Constraints
- 2 <= k <= 9
- 1 <= n <= 60
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