216. Combination Sum III

216. Combination Sum III

這一題和 77. Combinations 與 39. Combination Sum 差不多，其實比 40. Combination II 簡單。

class Solution:
    def combinationSum3(self, k: int, n: int) -> List[List[int]]:
        ans = []
        def backtrack(curr, target, start):
            if target == 0:
                if len(curr) == k:
                    ans.append(list(curr))
                return
            if target < 0:
                return
            else:
                for i in range(start, 10):
                    curr.append(i)
                    backtrack(curr, target - i, i + 1)
                    curr.pop()
        backtrack([], n, 1)
        return ans