Merge pull request neetcode-gh#1125 from agnihotriketan/patch-39

Create 17-Letter-Combinations-of-a-Phone-Number.cs

csharp/17-Letter-Combinations-Of-A-Phone-Number.cs

@@ -0,0 +1,34 @@
+public class Solution {
+ public IList<string> LetterCombinations(string digits)
+ {
+ var lettersMap = new Dictionary<char, string>
+ {
+ {'2', "abc"},
+ {'3', "def"},
+ {'4', "ghi"},
+ {'5', "jkl"},
+ {'6', "mno"},
+ {'7', "pqrs"},
+ {'8', "tuv"},
+ {'9', "wxyz"}
+ };
+
+ var result = new List<string>();
+
+ if (!string.IsNullOrEmpty(digits))
+ Backtrack(result, digits, lettersMap, "", 0);
+
+ return result;
+ }
+
+ void Backtrack(List<string> result, string digits, Dictionary<char, string> lettersMap, string curString, int start)
+ {
+ if (curString.Length == digits.Length)
+ { result.Add(curString); return; }
+
+ foreach (var c in lettersMap[digits[start]])
+ {
+ Backtrack(result, digits, lettersMap, curString + c, start + 1);
+ }
+ }
+}