letter_combinations_of_a_phone_number.cpp

#include<iostream>
#include<vector>
using namespace std;

class Solution {
    private:
    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;
    }
};