Create 98-Validate-Binary-Search-Tree.cs

Author: notauserx

csharp/98-Validate-Binary-Search-Tree.cs

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+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public int val;
+ *     public TreeNode left;
+ *     public TreeNode right;
+ *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+public class Solution {
+    public bool IsValidBST(TreeNode root, int? left = null, int? right = null) {
+        if(root == null) return true;
+        
+        if((left != null && root.val <= left) || 
+           (right != null && root.val >= right))
+           return false;
+        
+        return IsValidBST(root.left, left, root.val) &&
+            IsValidBST(root.right, root.val, right);
+    }
+}