validate_binary_tree.py

class Solution:
    
    def _bst_helper(self, root, lower, upper):
        if root is None:
            return True
        
        def within_contstraints(val):
            if lower is None and upper is None:
                return True
            if lower is None:
                return val < upper
            if upper is None:
                return val > lower
            return lower < val < upper
        
        this_val = root.val
        left_val = root.left.val if root.left else None
        right_val = root.right.val if root.right else None
        
        if left_val is not None and (left_val >= this_val or not within_contstraints(left_val)):
            return False
        
        if right_val is not None and (right_val <= this_val or not within_contstraints(right_val)):
            return False
        
        return self._bst_helper(root.left, lower, this_val) and self._bst_helper(root.right, this_val, upper)
    
    def isValidBST(self, root: TreeNode) -> bool:
        return self._bst_helper(root, None, None)