17. Letter Combinations of a Phone Number {Medium}

笛卡尔乘积 Cartesian product

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.

image.png

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.

reletreby's solution，apply itertools

from itertools import product
class Solution:
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if len(digits)==0:
            return []
        phone = {'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']}
                
        comb_list = []
        for d in digits:
            comb_list.append(phone[d])
        all_combs = product(*comb_list)
        return [''.join(c) for c in all_combs]

official solution:

class Solution:
    def __init__(self):
        self.phone = {'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']}
        self.output = []
        
    def backtrack(self, combination, next_digits):
        if len(next_digits) == 0:
            self.output.append(combination)
        else:
            for letter in self.phone[next_digits[0]]:
                self.backtrack(combination + letter, next_digits[1:])
                
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """               
        if digits:
            self.backtrack('', digits)
        return self.output