Letter Combinations of a Phone Number

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"].

class Solution {public:    void dfs(string table[],vector<string>&vec,int dep, int max, string &s, string cur){        string str = table[s.at(dep) - '2'];for(int i = 0; i < str.length(); i++){            string next = cur + str.at(i);            if(dep == max - 1){                vec.push_back(next);            }            else{                dfs(table,vec,dep + 1, max, s, next);            }        }    }    vector<string> letterCombinations(string digits) {        vector<string>vec;        if(digits.length() <= 0){            vec.push_back("");            return vec;        }string table[8] = {"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};            dfs(table,vec,0,digits.size(),digits,"");        return vec;    }    };