# 0501. Find Mode in Binary Search Tree

<https://leetcode.com/problems/find-mode-in-binary-search-tree>

## Description

Given the `root` of a binary search tree (BST) with duplicates, return *all the* [*mode(s)*](https://en.wikipedia.org/wiki/Mode_\(statistics\)) *(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:**

![](https://assets.leetcode.com/uploads/2021/03/11/mode-tree.jpg)

```
**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).

## ac

```java
```
