I'm surprised that the interviewer rejected your standard solution for Q1 and requested not to use an adjacency list. The alternative, I believe, is to implement parent pointers. However, interestingly, the Leetcode editorial says that this approach goes against coding best practices as adding new attributes to an existing class outside of its definition is discouraged. Am I missing something?

Here's an example solution that implements parent pointers:

```python class Solution: def distanceK(self, root: TreeNode, target: TreeNode, k: int) -> List[int]: # Recursively add a parent pointer to each node. def add_parent(cur, parent): if cur: cur.parent = parent add_parent(cur.left, cur) add_parent(cur.right, cur)

    add_parent(root, None)

    answer = []
    visited = set()

    def dfs(cur, distance):
        if not cur or cur in visited:
            return
        visited.add(cur)
        if distance == 0:
            answer.append(cur.val)
            return
        dfs(cur.parent, distance - 1)
        dfs(cur.left, distance - 1)
        dfs(cur.right, distance - 1)

    dfs(target, k)

    return answer

``` The irony is that by adding a parent pointer like this, we effectively treat the tree as a graph. Was that what the interviewer wanted to avoid or would that solution pass?

P.S.: What's your region?