Problem URL: largest-bst-subtree

We will use a recursive function to check if the tree is a BST or not. If it is, we will return the size of the tree. If it's not, we will return the maximum size of the left and right subtrees.

class Solution:
    def largestBSTSubtree(self, root: TreeNode) -> int:
        def isBST(root, minVal, maxVal):
            if not root:
                return True
            if root.val <= minVal or root.val >= maxVal:
                return False
            return isBST(root.left, minVal, root.val) and isBST(root.right, root.val, maxVal)

        def size(root):
            if not root:
                return 0
            return 1 + size(root.left) + size(root.right)

        if not root:
            return 0

        if isBST(root, -math.inf, math.inf):
            return size(root)

        return max(self.largestBSTSubtree(root.left), self.largestBSTSubtree(root.right))

Time complexity: O(n^2)
Space complexity: O(n)