My approach was to run a DFS first, collect all the leaves into an array, then loop through and wire up the next pointers.
Clarify the problem constraints (e.g., whether the tree is static or dynamic, and if the next pointers should be set in-place). Then, propose a DFS traversal that identifies leaf nodes and links them in order, using a previous pointer to connect each leaf to the next. Discuss time and space complexity, and consider iterative vs recursive approaches.
Pro tip: Mention that you can avoid extra space by using the tree's existing structure (e.g., Morris traversal) if recursion depth is a concern, but be prepared to discuss trade-offs between simplicity and efficiency.
Ask about tree size, recursion limits, and whether the next pointers are initially null. Confirm that leaves should be connected in DFS order (pre-order, in-order, or post-order?).
Decide between recursive and iterative DFS. Recursive is simpler but may cause stack overflow; iterative with explicit stack avoids that but uses extra space.
During traversal, when a leaf is encountered, link the previous leaf's next pointer to the current leaf, then update the previous pointer. Initialize previous as null and handle the first leaf.
State time complexity O(n) and space complexity O(h) for recursion or O(n) for iterative. Discuss edge cases: empty tree, single node, skewed tree, and multiple leaves.
If asked for O(1) space, propose Morris traversal to find leaves in order without extra space, but note it modifies the tree temporarily.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.