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);
  }

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