Leetcode17 : Letter Combinations of a Phone Number Solution

Problem statement could be found @https://leetcode.com/problems/letter-combinations-of-a-phone-number/

This code is very simple and just works on the recursion.

Approach -
1. Create a mapping of digits and corresponding characters
2. For each digit given in the input,  pick  one character at a time from the mapping and then move to next digit.

Code snippet -

class Solution {
public:
    map<char, string> dict;
    string digits;
    vector<string > retvec;
    void init(){
        dict[2] = "abc";
        dict[3] = "def";
        dict[4] = "ghi";
        dict[5] = "jkl";
        dict[6] = "mno";
        dict[7] = "pqrs";
        dict[8] = "tuv";
        dict[9] = "wxyz";
    }
    void helper( int index, string s){
        if ( index >= digits.length()){
            retvec.push_back(s);
            return ;
        }
        string x = dict[digits[index]-'0'];
        for ( int i=0;i<x.length();i++){
            helper(index+1, s+ x[i]);
           
        }
    }
    vector<string> letterCombinations(string digits) {
        this-> digits = digits;
        if ( digits.length() == 0){
            return retvec;
        }
        init();
        string s;
        helper(0, s);
        return retvec;
    }
};