I knew two pointers immediately but my brain kept defaulting to the ascending version I'd drilled a hundred times.
Clarify the problem constraints (descending order, in-place merge, extra capacity in first array) and then propose a two-pointer approach that fills the first array from the end to avoid overwriting. Walk through a small example to demonstrate correctness and discuss time/space complexity.
Pro tip: Emphasize that merging from the back is crucial for in-place operation and that this approach generalizes to ascending order by reversing the comparison. Mention that this is a common variation of the classic merge step in merge sort.
Restate the problem to ensure understanding: two descending arrays, first has extra capacity, merge second into first in-place, result descending. Ask about edge cases (empty arrays, duplicates).
Decide on a two-pointer approach starting from the end of both arrays and filling the first array from its end. Explain why this avoids overwriting elements.
Use a small example (e.g., A = [9,5,3,_,_], B = [8,4]) to illustrate pointer movement and placement. Show how the result is built from the back.
State time complexity O(m+n) and space complexity O(1). Discuss why this is optimal for in-place merging.
Mention handling of empty arrays, all elements of one array being larger, and duplicates. Compare with alternative approaches (e.g., using extra space) and justify the in-place method.
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