题目描述：

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.

　　

　　这道题不算难，想想就出来了。

solution：

vector
     
       letterCombinations(string digits) {    int n = digits.size();    vector
      
        res;    if(n == 0)        return res;    res.push_back("");    string numap[] = {
   "#","#","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};    for (int i = 0;i < n;++i)    {        vector
       
         tmp;        for(int j = 0;j < res.size();++j)            for(int k = 0;k < numap[digits[i]-'0'].size();++k)                tmp.push_back(res[j] + numap[digits[i]-'0'][k]);        res = tmp;    }    return res;}
       
      
     

　　此题还有其他解法，《编程之美》中也有相似题目，暂时先这样了。