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# 501. Find Mode in Binary Search Tree

## Description

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, 10`

.^{4}] `-10`

^{5}<= Node.val <= 10^{5}

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