Update 0700.二叉搜索树，添加C#

problems/0700.二叉搜索树中的搜索.md

@@ -464,6 +464,28 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+// 递归
+public TreeNode SearchBST(TreeNode root, int val)
+{
+    if (root == null || root.val == val) return root;
+    if (root.val > val) return SearchBST(root.left, val);
+    if (root.val < val) return SearchBST(root.right, val);
+    return null;
+}
+//  迭代
+public TreeNode SearchBST(TreeNode root, int val)
+{
+    while (root != null)
+    {
+        if (root.val > val) root = root.left;
+        else if (root.val < val) root = root.right;
+        else return root;
+    }
+    return null;
+}
+```
 
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