# Leetcode 216 - Combination sum III

Note:

• This is a backtracking problem even though it can be solve using dp, but it’s kinda hard.

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: