LeetCode Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.

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 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public boolean isValidBST(TreeNode root) {
12         return checkBST(root,Long.MIN_VALUE,Long.MAX_VALUE);
13     }
14 
15     private boolean checkBST(TreeNode node,long leftval,long rightval){
16         if (node==null) return true;
17         return leftval<node.val && node.val<rightval && checkBST(node.left,leftval,node.val)
18                 && checkBST(node.right,node.val,rightval);
19 
20     }
21 }