leetcode#17. Letter Combinations of a Phone Number

class Solution {
public:

    vector<string> letterCombinations(string digits)
    {
        if(digits.empty()) return {};
        vector<string> result;
        result=letterCombinations(digits,0);
        return result;       
    }
　　//是一个递归，如果digits是123那么我只要构造23，然后每个元素都加上1对应的三个字母即可
     vector<string> letterCombinations(string& digits,int index)//构建从index~digit.size()-1
    {
         if(index>=digits.size()) return {};
         auto str=col[string()+digits[index]];
         vector<string> result;
         for(int i=0;i!=str.size();++i)
         {
            string re;
            auto vec=letterCombinations(digits,index+1);
            if(vec.empty())
                result.push_back(string()+str[i]);
            else
            {
                 for(auto & elem:vec)
                {
                    auto temp=string()+str[i]+elem;
                    result.push_back(temp);
                }
            }           
         }
         return result;           
    }
    
    
private:
//这里第一个string可以改成char....
    unordered_map<string,string> col={{"2","abc"},{"3","def"},{"4","ghi"},{"5","jkl"},{"6","mno"},{"7","pqrs"},{"8","tuv"},{"9","wxyz"}};
};