# [LeetCode] 501. Find Mode in Binary Search Tree

## Problem

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 .

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).

## Solution

``````class Solution {
int count = 0;
int maxCount = 0;
Integer pre = null;
public int[] findMode(TreeNode root) {
List<Integer> resList = new ArrayList<>();
helper(root, resList);
return resList.stream().mapToInt(i->i).toArray();
}
private void helper(TreeNode root, List<Integer> res) {
if (root == null) return;
helper(root.left, res);
if (pre == null) {
pre = root.val;
count = 1;
maxCount = 1;
} else {
if (root.val == pre) {
count++;
if (count > maxCount) {
res.clear();
maxCount = count;
}
} else {
pre = root.val;
count = 1;
}
if (count == maxCount) {