```
class Solution(object):
def letterCombinations(self, digits):
d = {'2':'abc', '3':'def', '4':'ghi', '5':'jkl', '6':'mno', '7':'pqrs', '8':'tuv', '9':'wxyz'}
return [''.join(i) for i in itertools.product(*(d[k] for k in digits))] if len(digits) > 0 else []
```