Leetcode 17. 电话号码的字母组合 （dfs）

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

class Solution {
public:
    vector<string>ans; 
    map<char, string> M = {
        {'2', "abc"}, {'3', "def"}, {'4', "ghi"}, {'5', "jkl"}, {'6', "mno"},
        {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"}
    };
    vector<string> letterCombinations(string digits) {
       
        if(digits=="")return ans;
        dfs(digits,0,"");
        
        return ans;
    }
    inline void dfs(string digits,int k,string tep)
    {
        if(k==digits.size())
        {
            ans.push_back(tep);
            
            return ;
        }
        string p=M[digits[k]];
        for(int i=0;p[i];i++)
        {
            tep+=p[i];
            dfs(digits,k+1,tep);
            tep.erase(tep.end()-1);
        }
    }
};