[LeetCode] 17. Letter Combinations of a Phone Number 电话号码的字母组合 Given a string containing digits from 2-9inclusive, 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"].

class Solution {public:vector letterCombinations(string digits) {if (digits.empty()) return {}; vector res; vector dict{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"}; letterCombinationsDFS(digits, dict, 0, "", res); return res; }void letterCombinationsDFS(string& digits, vector& dict, int level, string out, vector& res) {if (level == digits.size()) {res.push_back(out); return; }string str = dict[digits[level] - '0']; for (int i = 0; i < str.size(); ++i) {letterCombinationsDFS(digits, dict, level + 1, out + str[i], res); }}};

class Solution {public:vector letterCombinations(string digits) {if (digits.empty()) return {}; vector res{""}; vector dict{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"}; for (int i = 0; i < digits.size(); ++i) {vector t; string str = dict[digits[i] - '0']; for (int j = 0; j < str.size(); ++j) {for (string s : res) t.push_back(s + str[j]); }res = t; }return res; }};