working solution

[avcode3]

No description provided.

[super30admin]

Your solution for the Validate BST problem is on the right track. You are using a recursive approach that checks each node against allowed min and max values, which is a standard method. However, there are a few issues to fix:

• There is a syntax error in float(-inf). It should be float('-inf') or -float('inf'). Similarly, float(inf) should be float('inf').
• The use of self.flag is acceptable, but consider optimizing by returning early when the tree is found to be invalid. Currently, even after setting self.flag to false, the recursion continues. You can add checks to stop the recursion once the flag is false.
• Alternatively, you can avoid using an instance variable by having the helper function return a boolean. This would make the code more modular and avoid side effects.

Here is a revised version of your code that addresses these issues:

class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        return self.helper(root, float('-inf'), float('inf'))
    
    def helper(self, node, min_val, max_val):
        if not node:
            return True
        if node.val <= min_val or node.val >= max_val:
            return False
        return self.helper(node.left, min_val, node.val) and self.helper(node.right, node.val, max_val)

This version is cleaner and avoids the use of an instance variable. It also stops the recursion as soon as an invalid node is found.