update538.把二叉搜索树转换为累加树，添加C#

problems/0538.把二叉搜索树转换为累加树.md

@@ -529,6 +529,23 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+// 递归
+public class Solution
+{
+    int pre = 0;
+    public TreeNode ConvertBST(TreeNode root)
+    {
+        if (root == null) return null;
+        ConvertBST(root.right);
+        root.val += pre;
+        pre = root.val;
+        ConvertBST(root.left);
+        return root;
+    }
+}
+```
 
 
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