Validate Binary Search Tree

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 a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isValidBST(TreeNode root) {
if (root == null)
return true;
return isSubtreeLessThan(root.left, root.val) && isSubtreeGreaterThan(root.right, root.val) && isValidBST(root.left) &&
isValidBST(root.right);
}

private boolean isSubtreeLessThan(TreeNode root, int value) {
if (root == null)
return true;

return root.val < value && isSubtreeLessThan(root.left, value) && isSubtreeLessThan(root.right, value);
}

private boolean isSubtreeGreaterThan(TreeNode root, int value) {
if (root == null)
return true;

return root.val > value && isSubtreeGreaterThan(root.left, value) && isSubtreeGreaterThan(root.right, value);
}
}