**Why Merge Sorted Arrays:** Mergesort's merge step, merge K sorted streams (logs from multiple sources), and combining partitioned data.
**Two Arrays:** Two pointers, O(m+n). K arrays: (1) Merge pairwise—O(N log K). (2) Heap of (value, stream_id)—O(N log K). Python: heapq.merge(a, b, c)—lazy, memory-efficient for large streams.
**Scalability:** heapq.merge doesn't load all into memory—streams. For Spark: merge sorted part files via sortMerge join....
The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations. It covers performance optimization strategies, common pitfalls to avoid, and real-world examples from companies like Pubmatic. The answer also includes follow-up discussion points that interviewers commonly explore.
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