Find Mode in Binary Search Tree

Given the `root` of a binary search tree (BST) with duplicates, return _all the mode(s) (i.e., the most frequently occurred element) in it_. If the tree has more than one mode, return them in **any order**. Assume a BST is defined as follows: - The left subtree of a node contains only nodes with keys **less than or equal to** the node's key. - The right subtree of a node contains only nodes with keys **greater than or equal to** the node's key. - Both the left and right subtrees must also be binary search trees.   **Example 1:** **Input:** root = [1,null,2,2] **Output:** [2] ``` **Example 2:** **Input:** root = [0] **Output:** [0] ```   **Constraints:** - The number of nodes in the tree is in the range `[1, 104]`. - `-105 <= Node.val <= 105`   **Follow up:** Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).