LeetCode 17. Letter Combinations of a Phone Number

void DFS(int pos,string di,string temp,vector<string> &ans){
    if (pos==0) {ans.push_back(temp);return;}
    string m[8] = {"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    int len=m[di[0]-‘2‘].length();
    while(len--){
        DFS(pos-1,di.substr(1,pos-1),temp+m[di[0]-‘2‘][len],ans);
    }
}

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        int len=digits.length();
        if(digits.empty()) return vector<string>();
        vector<string> ans;
        DFS(len,digits+" ","",ans);
        return ans;
    }
};