Letter Combinations of a Phone Number

 Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

 

Example 1:

Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]

Example 2:

Input: digits = ""
Output: []

Example 3:

Input: digits = "2"
Output: ["a","b","c"] 

c++ implementation:

  vector<string> letterCombinations(string digits) 
  {
                vector<string> result ;
        if (digits.length() == 0) return result;
        
        string dict[10]= {"0", "1", "abc", "def", "ghi", "jkl", 
         "mno", "pqrs", "tuv","wxyz"};
      
        backtrack(result, digits,"", dict);
        return result; 
  }
    
    
 void backtrack(vector<string>&result,string digits, string cur, 
string dict[]) 
  {
        if (cur.length() == digits.length()) 
        { 
            result.push_back(cur); 
            return; 
        }
     
        int index=cur.length();
            
        string letter=dict[digits[index]-'0'];
            
        for (int i=0;i<letter.length();i++) 
        {
            
         backtrack(result, digits, cur+letter[i] , dict);
        }
  
  }