17 Letter Combinations of a Phone number

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        ans = []
        d = {   2:"abc",
                3:"def",
                4:"ghi",
                5:"jkl",
                6:"mno",
                7:"pqrs",
                8:"tuv",
                9:"wxyz"
            }
        for i in range(len(digits)):
            p = d[ord(digits[i])-ord("0")]
            if len(ans) == 0:
                for j in p:
                    ans.append(j)
            else:
                ans_new = []
                for j in p:
                    for k in ans:
                        ans_new.append(k+j)
                ans = ans_new
            #print ans
        return ans