Given a binary tree root, the task is to return the maximum sum of all keys of any sub-tree which is also a Binary Search Tree (BST). Assume a BST is defined as follows: The left subtree of a node contains only nodes with keys less than the node's key. The right subtree of a node contains only nodes with keys greater than the node's key. Both the left and right subtrees must also be binary search trees. Example 1: Input: root = [1,4,3,2,4,2,5,null,null,null,null,null,null,4,6] Output: 20 Explanation: Maximum sum in a valid Binary search tree is obtained in root node with key equal to 3. Example 2: Input: root = [4,3,null,1,2] Output: 2 Explanation: Maximum sum in a valid Binary search tree is obtained in a single root node with key equal to 2. Example 3: Input: root = [-4,-2,-5] Output: 0 Explanation: All values are negatives. Return an empty BST. Example 4: Input: root = [2,1,3] Output: 6 Example 5: Input: root = [5,4,8,3,null,6,3] Output: 7 Constraints: The given binary tree will have between 1 and 40000 nodes. Each node's value is between [-4 * 10^4 , 4 * 10^4].