LC.98.Validate Binary Search Tree

https://leetcode.com/problems/validate-binary-search-tree/description/
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.
Example 1:
    2
   / \
  1   3
Binary tree [2,1,3], return true.
Example 2:
    1
   / \
  2   3
Binary tree [1,2,3], return false.

note: by the definition, here it mentions:
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.
time: o(n)
space: o(n)

 1 public boolean isValidBST(TreeNode root) {
 2         if (root == null ) return true;
 3         return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
 4     }
 5 
 6     private boolean helper(TreeNode root, long minValue, long maxValue) {
 7         if (root == null) return true ;
 8         if (root.val >=maxValue || root.val <= minValue){
 9             return false ;
10         }
11         return helper(root.left, minValue, root.val) && helper(root.right, root.val, maxValue);
12     }