Leetcode Problem 17 Letter Combinations of a phone number.txt

Leetcode Problem 17 : Letter Combinations of a Phone Number
Description: Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.
Example : Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Hint to solve: recrusive solution until no digit left.

Language: C#


public class Solution 
{
    IList<string> Answer;
    List<string> mapping;
    public Solution()
    {
        Answer = new List<string>();
        mapping = new List<string>{"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    }
    
    public void GeneratePermutaion(string digitsLeft,string currentCombination)
    {
       // Console.WriteLine("DigitsLeft: "+ digitsLeft + " currentCombination: "+currentCombination);
       if(String.IsNullOrEmpty(digitsLeft))
       {
           Answer.Add(currentCombination);
           return;
       }
        
        char firstDigit = digitsLeft[0];
        string temp = "";
        for(int i = 1; i<digitsLeft.Count(); ++i)
        {
            temp += digitsLeft[i];
        }
        digitsLeft = temp;
        
        string alphabets = mapping[firstDigit - '0'];
        for(int i = 0; i<alphabets.Count(); ++i)
        {
            string combination = currentCombination + alphabets[i];
            GeneratePermutaion(digitsLeft,combination);
        }
        
    }
    public IList<string> LetterCombinations(string digits) 
    {
        if(String.IsNullOrEmpty(digits))
            return Answer;
        
        string combination = "";
        GeneratePermutaion(digits,combination);
        return Answer;
        
    }
}