Given the root of a binary tree, find the largest subtree, which is also a Binary Search Tree (BST), where the largest means subtree has the largest number of nodes. A Binary Search Tree (BST) is a tree in which all the nodes follow the below-mentioned properties: The left subtree values are less than the value of their parent (root) node's value. The right subtree values are greater than the value of their parent (root) node's value.