17. Letter Combination

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if not digits:
            return []
        
        keyboard = {
            "2": "abc",
            "3": "def",
            "4": "ghi",
            "5": "jkl",
            "6": "mno",
            "7": "pqrs",
            "8": "tuv",
            "9": "wxyz"
        }
        
        res = []
        curr = []

        def bt(i):
            if len(curr) == len(digits):
                res.append(''.join(curr))
                return
            digit = digits[i]
            letters = keyboard[digit]

            for char in letters:
                curr.append(char)
                bt(i+1)
                curr.pop()
        bt(0)
        return res