letters_combination_of_phone_number.cpp

class Solution {
private:
    void solve(string digits, int index,string output, vector<string>& ans, string *mapping){
        //base case
        if(index>=digits.size()){
            ans.push_back(output);
            return;
        }
        
        int number = digits[index] - '0';
        string value = mapping[number];
        for(int i=0; i<value.size(); i++){
            output.push_back(value[i]);
            solve(digits,index+1,output,ans,mapping);
            output.pop_back();
        }
    }
public:
    vector<string> letterCombinations(string digits) {
        vector<string> ans;
        if(digits.size()==0) return ans;
        string output="";
        int index = 0;
        string mapping[10]={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        solve(digits,index,output, ans, mapping);
        return ans;
    }
};