Given the `root` of a binary tree, return the most frequent subtree sum. If there is a tie, return all the values with the highest frequency in any order.

The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself).

Example 1:

    Input: root = [5,2,-3]
    Output: [2,-3,4]

Example 2:

    Input: root = [5,2,-5]
    Output: [2]

Constraints:

- The number of nodes in the tree is in the range `[1, 104]`.
- `-105 <= Node.val <= 105`