Leetcode 17. Letter Combinations of a Phone Number

Problem Link

In this problem we will be given a string of digits, as like old days featured phones. We have to generate the number of ways we can generate string if,

digit 2 = "abc"

digit 3 = "def"

....

digit 9 = "wxyz"

Solution idea:
we can use dfs  approach for solve this.

Code: 

 class Solution {  
 public:  
   map<char,string>mm;  
   vector<string>vec;  
   void solve(string digits,int pos,string ans){  
     if(pos == digits.size()){  
       if(digits!="")vec.push_back(ans);  
       return;  
     }  
     for(auto it:mm[digits[pos]]){  
       ans.push_back(it);  
       solve(digits,pos+1,ans);  
       ans.pop_back();  
     }      
   }  
   vector<string> letterCombinations(string digits) {  
     mm['2'] = "abc";  
     mm['3'] = "def";  
     mm['4'] = "ghi";  
     mm['5'] = "jkl";  
     mm['6'] = "mno";  
     mm['7'] = "pqrs";  
     mm['8'] = "tuv";  
     mm['9'] = "wxyz";  
     solve(digits,0,"");  
     return vec;  
   }  
 };