# 输出:["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
def letterCombinations(self, digits: str) -> List[str]:
mapper = [" ", " ", "abc", "def", "ghi",
"jkl", "mno", "pqrs", "tuv", "wxyz"]
def backtrack(digits, start):
for i in range(start, len(digits)):
for c in mapper[int(digits[i])]:
for p in backtrack(digits, i + 1):
# 需要过滤诸如 "d", "e", "f" 等长度不符合的数据
if len(c + p) == len(digits):
return backtrack(digits, 0)