Two-pointer sweep wasn't hard to get to, the intersection formula is just max of starts and min of ends, check that max <= min and you're done.
Use a two-pointer technique to traverse both sorted lists, comparing intervals and advancing the pointer with the smaller end point. For the follow-up, discuss external sorting or streaming with chunked processing to handle memory constraints.
Pro tip: Clarify interval representation (inclusive/exclusive) and edge cases like touching intervals before coding; this shows attention to detail and prevents off-by-one errors.
Confirm interval definition (closed, inclusive), sorted order, and disjointness. Discuss edge cases like empty lists, single intervals, and touching intervals.
Initialize pointers at the start of each list. While both pointers are valid, compute intersection if intervals overlap, then advance the pointer with the smaller end value.
Explain O(m+n) time and O(1) extra space. Argue correctness by invariant: pointers only advance past intervals that cannot intersect future intervals.
For large lists, propose external sorting or streaming: read chunks, merge-sort on disk, then apply two-pointer with buffered I/O. Alternatively, use a distributed approach if data is sharded.
Compare in-memory vs. external approaches. Mention binary search for skewed sizes, and consider parallel processing for independent chunks.
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