Update 0968.监控二叉树，添加C#

Author: eeee0717

problems/0968.监控二叉树.md

@@ -726,6 +726,31 @@ impl Solution {
     }
 }
 
+```
+### C#
+```csharp
+public class Solution
+{
+    public int res = 0;
+    public int MinCameraCover(TreeNode root)
+    {
+        if (Traversal(root) == 0) res++;
+        return res;
+    }
+    public int Traversal(TreeNode cur)
+    {
+        if (cur == null) return 2;
+        int left = Traversal(cur.left);
+        int right = Traversal(cur.right);
+        if (left == 2 && right == 2) return 0;
+        else if (left == 0 || right == 0)
+        {
+            res++;
+            return 1;
+        }
+        else return 2;
+    }
+}
 ```
 
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