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Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.

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.

For example:

Given BST `[1,null,2,2]`

,

1 \ 2 / 2

return `[2]`

.

**Note:**
If a tree has more than one mode, you can return them in any order.

**Follow up:**
Could you do that without using any extra space? (Assume that the implicit stack space incurred due to recursion does not count).

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