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.
Within each recursion, set a lower and upper bound of the node value according to the definition. If node value is not within the bound, return false. 

/**
* 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 helper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
}

private boolean helper(TreeNode root, int min, int max) {
if (root == null) {
return true;
}

if (root.val <= min || root.val >= max) {
return false;
}

return helper(root.left, min, Math.min(max, root.val)) && helper(root.right, Math.max(min, root.val), max);
}
}