I thought I had it after reading the problem.
Break the problem into three distinct traversals: left boundary (excluding leaves), all leaves (left-to-right), and right boundary (excluding leaves, collected bottom-up). Use DFS to collect nodes, ensuring no duplicates by carefully defining which nodes belong to each part. Then concatenate the results in the correct order.
Pro tip: Clarify with the interviewer whether the root should be included if it's also a leaf (i.e., tree with only root). Also, explicitly state that you'll avoid duplicates by not adding leaves during boundary traversals.
Check if the root is null; if so, return an empty list. Also, if the tree has only one node, return that node as the boundary. Define left boundary as nodes from root's left child down to the leftmost node, excluding leaves.
Traverse from the root's left child, preferring left child over right, and add nodes to the result if they are not leaves. Stop when you reach a leaf.
Perform a DFS (preorder) traversal of the entire tree, adding nodes that have no children (leaves) to the result in left-to-right order.
Traverse from the root's right child, preferring right child over left, and add nodes to a temporary list if they are not leaves. After traversal, reverse the list and append to the result.
Concatenate the root value (if not already included), left boundary, leaves, and reversed right boundary. Ensure no duplicates by not adding leaves in boundary traversals.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.