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0098-validate-binary-search-tree/0098-validate-binary-search-tree.java

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+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+    public boolean isValidBST(TreeNode root) {
+        return check(root, Long.MIN_VALUE, Long.MAX_VALUE);
+    }
+    public boolean check(TreeNode node, long min, long max) {
+        if(node == null) return true;
+         if (node.val <= min || node.val >= max) {
+            return false;
+        }
+        return check(node.left, min, node.val) && 
+        check(node.right, node.val, max);
+    } 
+}