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.

confused what

"{1,#,2,3}"

means? > read more on how binary tree is serialized on OJ.

/**
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isValidBST(TreeNode root) {
return checkBST(root,Long.MIN_VALUE,Long.MAX_VALUE);
}

private boolean checkBST(TreeNode node,long leftval,long rightval){
if (node==null) return true;
return leftval<node.val && node.val<rightval && checkBST(node.left,leftval,node.val)
&& checkBST(node.right,node.val,rightval);

}
}