All Nodes Distance K In A Binary Tree - Performing Bidirectional Search On A Tree Using A Hashtable

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Question: You are given 3 things. The root of a binary tree, a single start node in the binary tree, and a number k. Return all nodes that are k "hops" away from the start node in the binary tree. Return a list of the values held at those nodes.
Complexities
n = total amount of nodes in the binary tree
m = total edges
Time: O( n + m )
This is standard to Breadth First Search. We upper bound the time by the number of nodes we can visit and edges we can traverse (at maximum).
Space: O( n )
We have a hashtable upper bounded by n mappings, a mapping to each node's parent.
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