Update 0501.二叉搜索树中的众数，添加C#版

problems/0501.二叉搜索树中的众数.md

@@ -1009,6 +1009,46 @@ pub fn find_mode(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
         res
     }
 ```
+### C#
+```C#
+// 递归 
+public class Solution
+{
+    public List<int> res = new List<int>();
+    public int count = 0;
+    public int maxCount = 0;
+    public TreeNode pre = null;
+    public int[] FindMode(TreeNode root)
+    {
+        SearchBST(root);
+        return res.ToArray();
+    }
+    public void SearchBST(TreeNode root)
+    {
+        if (root == null) return;
+        SearchBST(root.left);
+        if (pre == null)
+            count = 1;
+        else if (pre.val == root.val)
+            count++;
+        else
+            count = 1;
+
+        pre = root;
+        if (count == maxCount)
+        {
+            res.Add(root.val);
+        }
+        else if (count > maxCount)
+        {
+            res.Clear();
+            res.Add(root.val);
+            maxCount = count;
+        }
+        SearchBST(root.right);
+    }
+}
+```
 
 
 <p align="center">