leetcode_017 Letter Combinations of a Phone Number

Posted 2020-09-18

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"].

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直接解法：
Python实现

import copy
class Solution:
    # @param {string} digits A digital string
    # @return {string[]} all posible letter combinations
    def letterCombinations(self, digits):
        chr = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"]
        res = []
        for i in range(0, len(digits)):
            num = int(digits[i])
            tmp = []
            for j in range(0, len(chr[num])):
                if len(res):
                    for k in range(0, len(res)):
                        tmp.append(res[k] + chr[num][j])
                else:
                    tmp.append(chr[num][j])
            res = copy.copy(tmp)
        return res

 

dfs方法：还没有看

class Solution:
    # @return a list of strings, [s1, s2]
    def letterCombinations(self, digits):
        def dfs(num, string, res):
            if num == length:
                res.append(string)
                return
            for letter in dict[digits[num]]:
                    dfs(num+1, string+letter, res)
        
        dict = {‘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‘]
                }
        res = []
        length = len(digits)
        if length == 0:
            return res
        dfs(0, ‘‘, res)
        return res