代码训练营第二十五天（Python）｜ 216.组合总和III 、17.电话号码的字母组合

216.组合总和III

class Solution:
    def combinationSum3(self, k: int, n: int) -> List[List[int]]:
        res = []
        self.tracebacking(n, k, 1, 0, [], res)
        return res

    def tracebacking(self, targetsum, k, start, now_sum, path, res):
        if now_sum > targetsum:
            return

        if len(path) == k:
            if targetsum == now_sum:
                res.append(path[:])
            return

        for i in range(start, 9-(k-len(path))+2):
            path.append(i)
            now_sum += i
            self.tracebacking(targetsum, k, i+1, now_sum, path, res)
            now_sum -= i
            path.pop()

17.电话号码的字母组合

class Solution:

    num_dict = {
            2: "abc",
            3: "def",
            4: "ghi",
            5: "jkl",
            6: "mno",
            7: "pqrs",
            8: "tuv",
            9: "wxyz"
        }

    def letterCombinations(self, digits: str) -> List[str]:
        res = []
        if not digits:
            return res
        self.tracebacking(digits, 0, [], res)
        return res


    def tracebacking(self, digits, start_index, path, res):
        if len(path) == len(digits):
            res.append("".join(path[:]))
            return

        digit_int = int(digits[start_index])
        letters = self.num_dict.get(digit_int)
        for letter in letters:
            path.append(letter)
            self.tracebacking(digits, start_index+1, path, res)
            path.pop()