0501.find-mode-in-binary-search-tree

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:

1. The left subtree of a node contains only nodes with keys less than or equal to the node's key.
2. The right subtree of a node contains only nodes with keys greater than or equal to the node's key.
3. 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).

解题思路

见程序注释