17. Letter Combinations of a Phone Number

class Solution {
    void solve(string digit,string output,int index, vector<string>& ans,string mapping[])
    {
        //base case
        if(index >= digit.length())
        {
            ans.push_back(output);
            return ;

        }

        int number = digit[index] - '0';
        string value = mapping[number];

        for(int i =0 ;i<value.length();i++)
        {
            output.push_back(value[i]);
            solve(digit,output,index+1,ans,mapping);
            output.pop_back();
        }
    }
public:
    vector<string> letterCombinations(string digits) {
        
        vector<string> ans;
        if(digits.length()==0)
           return ans;
        string output;
        int index =0;

        string mapping[10] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};

        solve(digits,output,index,ans,mapping);

        return ans;
    }
};