Update 0669.修剪二叉搜索树，添加C#

problems/0669.修剪二叉搜索树.md

@@ -567,6 +567,23 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+// 递归
+public TreeNode TrimBST(TreeNode root, int low, int high)
+{
+    if (root == null) return null;
+    if (root.val < low)
+        return TrimBST(root.right, low, high);
+
+    if (root.val > high)
+        return TrimBST(root.left, low, high);
+
+    root.left = TrimBST(root.left, low, high);
+    root.right = TrimBST(root.right, low, high);
+    return root;
+}
+```
 
 
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