17. Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

class Solution {
public:
 
    string digit_lookuptable[10];
 

    vector<string> letterCombinations(string digits) {
        digit_lookuptable[0] = " ";        
        digit_lookuptable[1] = "OO";  
        digit_lookuptable[2] = "abc";
        digit_lookuptable[3] = "def";        
        digit_lookuptable[4] = "ghi";  
        digit_lookuptable[5] = "jkl";
        digit_lookuptable[6] = "mno";        
        digit_lookuptable[7] = "pqrs";  
        digit_lookuptable[8] = "tuv";
        digit_lookuptable[9] = "wxyz";
        
        vector<string>result;
        string ans;
        if(digits.size() == 0) return result;
        dfs(digits,0,ans,result);
        return result;
    }
    
    void dfs(string digits,int start,string &ans,vector<string>&result)
    {
       
        if(start == digits.size())
        {
            result.push_back(ans);
            return;
        }
        int curr_num = digits[start]-'0';
        for(int i = 0;i<digit_lookuptable[curr_num].size();i++)
         {
              ans = ans+digit_lookuptable[curr_num][i];
              dfs(digits,start+1,ans,result);
              ans.pop_back();
         }
    }
};