This problem is totally broken with Java 8. Boilerplate is hidden, but broken.

Very poor solution with the hidden/unviewable code for C++. You cannot include STL headers, you cannot use types you wish, any debug output breaks your results.

This task is terribad.

Sure seems that the Java 8 template is broken. You will be hit with a "Cannot find or load main class" Exception. Switch to Java 17 and you have to implement the boilerplate yourself. Just switch to Python and roll with it, if you're having trouble.

The code provided in python always print a "No" answer at the end, even if no revelant code is written. is It a bug?

An alternative solution I've made is to do an in order traversal of the tree. And since every subsequent node must be greater than the precedent node, any node that fails this condition means that the tree is not a valid binary search tree.

def checkBST(root):
  lastItem = None
  result = True
  
  def inOrderTraversal(node):
    nonlocal lastItem
    nonlocal result
    
    if node.left != None: inOrderTraversal(node.left)
  
    if lastItem != None and node.data <= lastItem: 
      result = False
    lastItem = node.data
    
    if node.right != None: inOrderTraversal(node.right)
  
  inOrderTraversal(root)
  return result