Find mode in a trinary search tree

You are given the root of a trinary search tree where each node has up to three children:

class Node {
 int val;
 Node left; // all values < val
 Node middle; // all values = val
 Node right; // all values > val
}

Assume the tree is intended to satisfy the ordering rules above.

Task

Return the mode(s) of the tree: the value(s) that occur most frequently among all nodes.

• If there is one mode, return a single-element list.
• If multiple values tie for highest frequency, return all such values (in any order).

Constraints (typical)

• Number of nodes n can be large (e.g., up to 10^5).
• Values can be any 32-bit signed integers.