17. Letter Combinations of a Phone Number

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

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"].

用个dictionary把数字和字母映射，然后recursion tree每层选一个字母，dfs

class Solution(object):
    def letterCombinations(self, digits):
        if not digits:
            return []
        dic = {"2":"abc", "3":"def", "4":"ghi", "5":"jkl", "6":"mno", "7":"pqrs", "8":"tuv", "9":"wxyz"}
        res=[]
        self.helper(0,digits,dic,res,[])
        return res
    
    def helper(self,index,digits,dic,res,cur):
        if len(cur)==len(digits):
            res.append("".join(cur))
            return
        for i in dic[digits[index]]:
            self.helper(index+1,digits,dic,res,cur+[i])