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问题：

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.

DFS

参考：http://www.cnblogs.com/Rosanna/p/3416949.html

代码：

class Solution {public:	    vector
   
     letterCombinations(string digits) {		string strlist[8] = {"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        string result = "";        m_digits = digits;		if(digits == "")		{			re_str.push_back("");			return re_str;		}        if(digits.size() == 0)            return re_str;        print(result, 0, strlist);        return re_str;    }    void print(string result, int i, string strlist[])    {        int num = m_digits[i] - '0';        if(i < m_digits.size())        {								for(int j = 0; j < strlist[num-2].size(); ++j)				{					print(result + strlist[num-2][j], i+1, strlist);				}        }        else        {            re_str.push_back(result);        }    }private:    vector
    
      re_str;    string m_digits;	};