Update 222.完全二叉树的节点个数，添加C#递归版

problems/0222.完全二叉树的节点个数.md

@@ -867,6 +867,31 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+// 递归
+public int CountNodes(TreeNode root)
+{
+    if (root == null) return 0;
+    var left = root.left;
+    var right = root.right;
+    int leftDepth = 0, rightDepth = 0;
+    while (left != null)
+    {
+        left = left.left;
+        leftDepth++;
+    }
+    while (right != null)
+    {
+        right = right.right;
+        rightDepth++;
+    }
+    if (leftDepth == rightDepth)
+        return (int)Math.Pow(2, leftDepth+1) - 1;
+    return CountNodes(root.left) + CountNodes(root.right) + 1;
+
+}
+```
 
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">