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 [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).

Solution

HashMap Solution with Java 8

public class Solution {
    public int[] findMode(TreeNode root) {
        if (root == null) return new int[0];
        Map<Integer, Integer> map = new HashMap<>();
        traversal(root, map);
        int max = map.entrySet().stream().max(Map.Entry.comparingByValue()).get().getValue();
        return map.entrySet().stream().filter(e -> e.getValue() == max).mapToInt(e -> e.getKey()).toArray();
    }

    private void traversal(TreeNode node, Map<Integer, Integer> map) {
        if (node == null) return;
        map.put(node.val, map.getOrDefault(node.val, 0) + 1);
        traversal(node.left, map);
        traversal(node.right, map);
    }
}

Analysis

Straightforward solution using HashMap
We simply get the frequency of each node.val
Then we get most frequent node.val with stream().max(Map.Entry.comparingByValue()).get()
At the end, we use filter() and mapToInt().toArray() to get the final int[] result