My first instinct was a hash set, which works fine but fails the space constraint.
Start by clarifying the problem and edge cases, then present a straightforward solution using hash sets to achieve O(m+n) time and O(m) space. For the follow-up, explain the two-pointer technique that aligns list lengths by switching pointers, achieving O(m+n) time and O(1) space.
Pro tip: Emphasize that intersection is by reference, not value, and proactively discuss edge cases like no intersection or one list being empty. This shows attention to detail and prevents incorrect assumptions.
Confirm that intersection means the same node reference, not just equal values. Ask about edge cases: empty lists, no intersection, or one list being a prefix of the other.
Propose using a hash set to store nodes of one list, then traverse the other to find the first common node. This takes O(m+n) time and O(m) space.
Introduce the two-pointer technique: traverse both lists simultaneously, and when a pointer reaches the end, redirect it to the head of the other list. They will meet at the intersection or both become null.
Explain that the two-pointer approach runs in O(m+n) time and O(1) space. Discuss how it handles different lengths and no intersection (both pointers become null after at most m+n steps).
Walk through a concrete example with lists of different lengths and an intersection, and another with no intersection, to verify the logic.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.