Python 8 lines simple solution


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    G
    def largestBSTSubtree(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        return self.dfs(root)[2]
        
    def dfs(self, root):
        # return isBST(boolean), range, count
        if not root:
            return True, (float('inf'), -float('inf')), 0
        
        left_bst, left_range, left_count = self.dfs(root.left)
        right_bst, right_range, right_count = self.dfs(root.right)
        
        if left_bst and right_bst and left_range[1] < root.val < right_range[0]:
                return True, (min(left_range[0], root.val), max(right_range[1], root.val)), left_count+right_count + 1
        return False, None, max(left_count, right_count)

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