【Leetcode】Letter Combinations of a Phone Number

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 public:
 3     vector<string> letterCombinations(string digits) {
 4         mapping = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
 5         vector<string> ret;
 6         letterCombinationsCore(digits, "", ret);
 7         return ret;
 8     }
 9     void letterCombinationsCore(string digits, string sub, vector<string> &ret) {
10         if (sub.size() == digits.size()) {
11             ret.push_back(sub);
12         } else {
13             int i = digits[sub.size()] - '0';
14             for (int j = 0; j < mapping[i - 2].size(); ++j) {
15                 sub.push_back(mapping[i - 2][j]);
16                 letterCombinationsCore(digits, sub, ret);
17                 sub.pop_back();
18             }
19         }
20     }
21     vector<string> mapping;
22 };

View Code

又是一个递归搜索题。