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.
public boolean isValidBST(TreeNode root) { // write your code here return isValidBSTHelper(root, Integer.MIN_VALUE, Integer.MAX_VALUE); } //return if the root val is within the [min,max] range public boolean isValidBSTHelper(TreeNode root, int min, int max) { if (root == null) { return true; } if (min > root.val || root.val > max) { return false; } return isValidBSTHelper(root.left, min, root.val - 1) && isValidBSTHelper(root.right, root.val + 1, max); }