We need to find largest BST in a binary tree. For that we need to tell that at a current node both left and right subtree is a binary tree or not and the current node is also a valid node.

• To do that we follow bottom up approach and maintain the minimum value found and the maximum value found while traversing up the tree.
• To find if the current node is a valid BST node or not, we see if the maximum value of subtree is smaller than current node and minimum value of right subtree is greater than the current node

def largestBst(self, root):
        maxAns = 0
        # returned values are [maxSize, minimum, maximum]
        def helper(root):
            if root is None:
                # this is particulary of a edge
                # where we need to statisfy the bst condition
                return (0, 1e10, -1e10)
            lans, lmini, lmaxi = helper(root.left)
            rans, rmini, rmaxi = helper(root.right)
            
            if lmaxi < root.data < rmini:
                return (lans + rans + 1, min(lmini, root.data), max(rmaxi, root.data))
            
            return (max(lans, rans), -1e10, 1e10)
        return helper(root)[0]