My first instinct was to iterate but recursion is just cleaner here.
Use a recursive depth-first traversal to merge the two trees simultaneously. At each step, if both nodes exist, sum their values and recursively merge their left and right children; if one is null, return the other subtree as-is. This naturally handles overlapping and non-overlapping nodes.
Pro tip: Clarify whether you can modify one of the input trees in place or must create a new tree, as this affects space complexity and is a common follow-up. Also, mention that the merge is commutative and associative, which can be a nice observation.
Ask about edge cases: can trees be empty? Should we mutate inputs or create a new tree? Are node values integers? Confirm that overlapping means nodes at the same position.
Write a function merge(t1, t2) that returns the merged subtree. Base cases: if t1 is null, return t2; if t2 is null, return t1.
If both nodes exist, create a new node with value t1.val + t2.val, then recursively set its left and right children by merging t1.left with t2.left and t1.right with t2.right.
Time complexity is O(min(n, m)) where n and m are the number of nodes in each tree, as we only traverse overlapping nodes. Space complexity is O(min(h1, h2)) for recursion stack, or O(min(n, m)) if creating a new tree.
Walk through a simple example, e.g., merging two trees with some overlapping and some non-overlapping nodes. Also test edge cases: one tree empty, both empty, trees of different shapes.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.