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.

OJ's Binary Tree Serialization:
The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.
Here's an example:

1
/ \
2   3
/
4
\
5

The above binary tree is serialized as 

"{1,2,3,#,#,4,#,#,5}"

.

/**
* 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) {
// Start typing your Java solution below
// DO NOT write main() function
int max = Integer.MAX_VALUE;
int min = Integer.MIN_VALUE;
return isBST(root, min, max);
}

public boolean isBST(TreeNode root, int min, int max){
if(root == null) {
return true;
}
if(root.val < max && root.val > min
&& isBST(root.left, min, root.val)
&& isBST(root.right, root.val, max)) {
return true;
}
else {
return false;
}
}
}