Update0017.电话号码的字母组合，添加C#

problems/0017.电话号码的字母组合.md

@@ -732,6 +732,38 @@ def backtracking(result, letter_map, digits, path, index)
   end
 end
 ```
+### C#
+```C#
+public class Solution
+{
+    public IList<string> res = new List<string>();
+    public string s;
+    public string[] letterMap = new string[10] { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
+    public IList<string> LetterCombinations(string digits)
+    {
+        if (digits.Length == 0)
+            return res;
+        BackTracking(digits, 0);
+        return res;
+    }
+    public void BackTracking(string digits, int index)
+    {
+        if (index == digits.Length)
+        {
+            res.Add(s);
+            return;
+        }
+        int digit = digits[index] - '0';
+        string letters = letterMap[digit];
+        for (int i = 0; i < letters.Length; i++)
+        {
+            s += letters[i];
+            BackTracking(digits, index + 1);
+            s = s.Substring(0, s.Length - 1);
+        }
+    }
+}
+```
 
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