```
class Solution:
# @return a list of strings, [s1, s2]
def letterCombinations(self, digits):
letter={'2':'abc','3':'def','4':'ghi','5':'jkl','6':'mno','7':'pqrs','8':'tuv','9':'wxzy'}
if digits=='':
return [""]
comb=list(letter[digits[0]])
for i in xrange(1,len(digits)):
comb=[x+y for x in comb for y in letter[digits[i]]]
return comb
```