using Microsoft.VisualStudio.TestTools.UnitTesting;

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace LeetCode

{

[TestClass]

public class Solution

{

private List<string> letterList = new List<string> { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

public IList<string> LetterCombinations(string digits)

{

IList<string> letterCombination = new List<string>();

this.LetterCombinationHelper(digits, digits.Length, new char[digits.Length], 0, letterCombination);

return letterCombination;

}

public void LetterCombinationHelper(string digits, int len, char[] output, int count, IList<string> letterCombination)

{

if (len == 0)

return;

if (len == count)

{

letterCombination.Add(new string(output));

System.Diagnostics.Debug.WriteLine(new string (output));

return;

}

int index = int.Parse(digits[count].ToString());

if (index == 0 || index == 1)

{

return;

}

for (int i = 0; i < letterList[index].Length; i++)

{

output[count ] = letterList[index][i];

this.LetterCombinationHelper(digits, len, output, count + 1, letterCombination);

}

}

[TestMethod]

public void TestLetterCombination()

{

IList<string> expectedList = new List<string> { "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf" };

IList<string> letterCombinationList = this.LetterCombinations("1");

Assert.AreEqual(expectedList.Count, letterCombinationList.Count);

foreach(string expectedStr in expectedList)

{

Assert.IsTrue(letterCombinationList.Contains<string>(expectedStr));

}

}

}

}