Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
Example 1:
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
Input: digits = ""
Output: []
Example 3:
Input: digits = "2"
Output: ["a","b","c"]
Solution
class Solution:
def letterCombinations(self, digits: str) -> List[str]:
lookup = {'2': 'abc',
'3': 'def',
'4': 'ghi',
'5': 'jkl',
'6': 'mno',
'7': 'pqrs',
'8': 'tuv',
'9': 'wxyz'}
out = list()
def backtrack(i, curr):
if len(digits) == len(curr):
out.append(curr)
return
for c in lookup[digits[i]]:
backtrack(i+1, curr + c)
if digits:
backtrack(0, '')
return out