A node is valid only if its value is strictly between a min_value and a max_value. When moving left, we update the max_value. When moving right, we update the min_value.

1. Define a helper function validate(node, min_val, max_val).
2. If node is None, return True.
3. If node.val <= min_val or node.val >= max_val, return False.
4. Check subtrees:
   • validate(node.left, min_val, node.val)
   • validate(node.right, node.val, max_val)
5. Return the logical AND of these checks.

• Time Complexity: O(N).
• Space Complexity: O(H).

def is_valid_bst(root):
    def validate(node, low=-float('inf'), high=float('inf')):
        if not node:
            return True
        if not (low < node.val < high):
            return False
        
        return (validate(node.left, low, node.val) and 
                validate(node.right, node.val, high))
                
    return validate(root)