update 二叉树理论基础.md 增加C#代码

Author: XueshanChen

problems/二叉树理论基础.md

@@ -302,9 +302,19 @@ impl<T> TreeNode<T> {
         }
     }
 }
-```
 
+```
+```c#
+public class TreeNode
+{
+    public int val;
+    public TreeNode left;
+    public TreeNode right;
+    public TreeNode(int x) { val = x; }
+}
+```
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>
 </a>
+