class Solution {
  public boolean isValidBST(TreeNode root) {
    return isValidBST(root, null, null);
  }

  private boolean isValidBST(TreeNode root, TreeNode minNode, TreeNode maxNode) {
    if (root == null)
      return true;
    if (minNode != null && root.val <= minNode.val)
      return false;
    if (maxNode != null && root.val >= maxNode.val)
      return false;

    return isValidBST(root.left, minNode, root) &&
           isValidBST(root.right, root, maxNode);
  }
}