17. Letter Combinations of a Phone Number

[fockspaces]

1. use backtracking method to record the string
2. update the ans when it reach the end (digits is empty)

class Solution:
    letters = {
        "2": "abc",
        "3": "def",
        "4": "ghi",
        "5": "jkl",
        "6": "mno",
        "7": "pqrs",
        "8": "tuv",
        "9": "wxyz"
    }
    def letterCombinations(self, digits: str) -> List[str]:
        ans = []
        def helper(digits, string):
            if not digits:
                return ans.append(string) if string else None
            remaining_digits = digits[1:]
            for letter in self.letters[digits[0]]:
                helper(remaining_digits, string + letter)
        helper(digits, "")
        return ans