```
def letterCombinations(self, digits):
"""
:type digits: str
:rtype: List[str]
"""
if digits =='':
return []
mapping = {'1':[''],
'2':['a','b','c'],
'3':['d','e','f'],
'4':['g','h','i'],
'5':['j','k','l'],
'6':['m','n','o'],
'7':['p','q','r','s'],
'8':['t','u','v'],
'9':['w','x','y','z'],
'0':[' ']}
res =['']
for d in digits:
temp = []
for item in res:
for x in mapping[d]:
temp.append(item+x)
res = temp
return res
```