【Leetcode】【Medium】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.

解题思路：

需要穷举不同中排列可能，典型需要回溯的思想；

可以选择递归或非递归的形式实现回溯；

递归：

 class Solution {
 public:
     vector<string> letterCombinations(string digits) {
         vector<string> lst;
         string ans;
         if (digits == "")
             return lst;
         Backtracking(lst, ans, digits, );
         return lst;
     }

     void Backtracking(vector<string> &lst, string ans, string digits, int idx) {
         if (idx == digits.size()) {
             lst.push_back(ans);
             return;
         }
         string cur_chars = d2l[digits[idx] - ''];
         for (int i = ; i < cur_chars.size(); ++i)
             Backtracking(lst, ans + cur_chars[i], digits, idx + );
     }

 private:
     vector<string> d2l = {
         " ", "", "abc", "def", "ghi", "jkl",
         "mno", "pqrs", "tuv", "wxyz"
     };
 };

非递归：