Update 0098.验证二叉搜索树，添加C#版

problems/0098.验证二叉搜索树.md

@@ -791,6 +791,20 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+// 递归
+public long val = Int64.MinValue;
+public bool IsValidBST(TreeNode root)
+{
+    if (root == null) return true;
+    bool left = IsValidBST(root.left);
+    if (root.val > val) val = root.val;
+    else return false;
+    bool right = IsValidBST(root.right);
+    return left && right;
+}
+```
 
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