LeetCode.501 Find Mode in Binary Search Tree

题目：

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).
分析：