Arrays, Intervals, Sliding Windows, And Prefix Sums
Asked of: Software Engineer
Last updated

What's being tested
Meta interviewers are probing linear-time array reasoning: transforming arrays without extra passes, counting contiguous ranges with prefix sums, and maintaining interval/window invariants under edge cases. You need to state constraints, choose the right template quickly, and defend O(n) or O(n log n) tradeoffs.
Patterns & templates
-
Prefix/suffix products for
productExceptSelf— two passes,O(n)time,O(1)extra space excluding output; handle one or multiple zeros. -
Prefix sum + hashmap for target-sum subarrays — maintain
count[prefix - k]; initialize{0: 1}; works with negatives unlike sliding window. -
Range-sum precomputation — build
prefix[i + 1] = prefix[i] + nums[i]; answersum(l,r)asprefix[r+1] - prefix[l]inO(1). -
Sliding window for longest transformable consecutive segment — expand right, track violation budget, shrink left until valid; usually
O(n). -
Interval gap scan for missing ranges — track previous boundary, compare
prev + 1tocurr - 1; guard empty input and integer limits. -
Order statistics for k-th largest — use min-heap size
kforO(n log k)or Quickselect averageO(n); clarify mutation.
Common pitfalls
Pitfall: Using sliding window for target-sum subarrays when numbers can be negative; use prefix-sum counts instead.
Pitfall: Forgetting boundary sentinels in missing ranges, especially empty arrays,
lower,upper, and 32-bit overflow aroundINT_MIN/INT_MAX.
Pitfall: Claiming
O(1)space forproductExceptSelfwhile allocating separate left and right arrays; only the output array may be excluded.
Practice these
The practice cards below cover the canonical variants — solve all of them and time yourself.
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