17. Letter Combinations of a Phone Number
Created: September 3, 2018 by [lek-tin]
Last updated: September 3, 2018
Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.
Example
Input: "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.
Solutions
DFS:
# Time - O(n * 4^n)
# Space - O(n * 4^n), max 4 possible chars per digit so O(4^n) strings each of length n
class Solution:
def letterCombinations(self, digits: str) -> List[str]:
if len(digits) == 0:
return []
if digits == None:
return []
lookup = {
'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']
}
results = []
s = ''
self.match(0, s, digits, lookup, results)
return results
def match(self, index, s, digits, lookup, results):
if (index == len(digits)):
results.append(s)
return
for char in lookup.get(digits[index]):
# Only pass a copy of s
self.match(index+1, s+char, digits, lookup, results)