添加 0541.反转字符串II C#版本

problems/0541.反转字符串II.md

@@ -272,9 +272,21 @@ func reverseStr(_ s: String, _ k: Int) -> String {
 }
 ```
 
-
-
-
+C#：
+```csharp
+public class Solution
+{
+    public string ReverseStr(string s, int k)
+    {
+        Span<char> span = s.ToCharArray().AsSpan();
+        for (int i = 0; i < span.Length; i += 2 * k)
+        {
+            span[i + k < span.Length ? i..(i + k) : i..].Reverse();
+        }
+        return span.ToString();
+    }
+}
+```
 
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