501. Find Mode in Binary Search Tree

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],

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).

private readonly Dictionary<int, int> dictionary = new Dictionary<int, int>();

        public int[] FindMode(TreeNode root)
        {
            Chuck(root);
            if (dictionary.Count > 0)
            {
                int max = dictionary.Max(x => x.Value);
                var array = dictionary.Where(x => x.Value == max).Select(x => x.Key).ToArray();
                return array;
            }
            else
            {
                return new int[0];
            }
        }

        private void Chuck(TreeNode node)
        {
            if (node == null)
            {
                return;
            }

            int val = node.val;
            if (dictionary.ContainsKey(val))
            {
                dictionary[val]++;
            }
            else
            {
                dictionary[val] = 1;
            }
            Chuck(node.left);
            Chuck(node.right);
        }