17. Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

do a dfs search to see all the combinations.
class Solution(object):
def letterCombinations(self, digits):
"""
:type digits: str
:rtype: List[str]
"""
NUMBER_HASH={'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']
}
digit_length = len(digits)
def dfs(number_length, string, result):
if number_length == digit_length:
result.append(string)
return
for letter in NUMBER_HASH[digits[number_length]]:
dfs(number_length + 1, string + letter, result)
result = []
if len(digits) == 0:
return result
dfs(0, '', result)
return result