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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.

每个数字代表一个种类，每个种类中有多个值，要求在每个种类中都选一个，所组成的所有不同的组合。

class Solution {public:   void dfs(vector
   
    & re, int cur_dep,string& cur_re, string digits){		static string tel_board[] = {			"0", "1", "abc", "def", "ghi", 			"jkl", "mno", "pqrs", "tuv", "wxyz", };		if(cur_dep == digits.length()){			re.push_back(cur_re);			return;		}		string cur_str = tel_board[digits[cur_dep] - '0'];		for (int i = 0; i < cur_str.length(); ++i)		{			cur_re.push_back(cur_str[i]);			dfs(re, cur_dep + 1, cur_re, digits);			cur_re.pop_back();		}			}	vector
    
      letterCombinations(string digits) {		vector
     
       re;		string cur_re("");		dfs(re, 0, cur_re, digits);		return re;	}};
     
    
   

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