Letter Combinations of a Phone N

//17. Letter Combinations of a Phone Number

#include <iostream>
#include <vector>
#include <string>
#include <cassert>

using namespace std;

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        res.clear();
        if(digits==""){
            return res;
        }
        findNext(digits,0,"");
        return res;
    }
private:
    vector<string> res;
    const string letterMaps[10] = {
            " ",    //0
            "",     //1
            "abc",  //2
            "def",  //3
            "ghi",  //4
            "jkl",  //5
            "mno",  //6
            "pqrs", //7
            "tuv",  //8
            "wxyz"  //9
    };
    void findNext(const string &digits,int index,const string &s){
        //const reference ,different const value

        if(index==digits.size()){
            res.push_back(s);
            //return can't miss
            return;
        }
        char c=digits[index];
        cout<<c<<endl;
        
        assert(c>='0' && c<='9' && c!='1');
        string letters=letterMaps[c-'0'];
        for(int i=0;i<letters.size();i++){
            findNext(digits,index+1,s+letters[i]);
        }
        return;
    }
};

int main(){

    
    vector<string> res=Solution().letterCombinations("34");
    for(int i=0;i<res.size();i++){
        cout<<res[i]<<" ";
    }
    cout<<endl;
    return 0;
}