class Solution:
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        phone = ['', '', 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz']
        ans = ['']
        for digit in map(int, digits):
            ans = [item + ch for item in ans for ch in phone[digit]]
        return ans if digits else []
class Solution:
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        phone = ['', '', 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz']
        ans = ['']
        for digit in map(int, digits):
            ans = [item + ch for item in ans for ch in phone[digit]]
        return ans if digits else []
:type digits: str         :rtype: List[str]