[LeetCode] 17. Letter Combinations of a Phone Number

Posted 2020-08-12

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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

 1 class Solution {
 2     unordered_map<char, vector<string>> ht  = {
 3              {‘0‘, {" "}}, {‘1‘, {""}}, {‘2‘, {"a", "b", "c"}}, {‘3‘, {"d", "e", "f"}},
 4              {‘4‘, {"g", "h", "i"}}, {‘5‘, {"j", "k", "l"}}, {‘6‘, {"m", "n", "o"}},
 5              {‘7‘, {"p", "q", "r", "s"}}, {‘8‘, {"t", "u", "v"}}, {‘9‘, {"w", "x", "y", "z"}},
 6     };       
 7 public:
 8     vector<string> letterCombinations(string digits) {
 9         vector<string> ret;
10         int n = digits.size();
11         
12         if (n == 0) return ret;
13     
14         if (n == 1) return ht[digits[0]];
15        
16         vector<string> prev = letterCombinations(digits.substr(0, n - 1));
17 
18         for (auto s : prev){
19             for (int i = 0; i < ht[digits[n-1]].size(); i++)
20             ret.push_back(s+ht[digits[n-1]][i]);
21         }
22         
23         return ret;
24     }
25 };