How do you find the minimal length of a contiguous subarray whose sum is at least target, for positive numbers?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Expand right to grow the sum, then shrink left while the sum stays at least target, tracking the shortest window.
WHY — Sliding Window instead of guessing?
Why interviewers care about Sliding Window:
question about Sliding Window.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Expand right to grow
the sum, then shrink left while the sum stays at least target, tracking the shortest window.
- 2Because values are positive,
left only moves forward: O(n) time and O(1) space.
- 3If negatives are allowed
this breaks and you need prefix sums plus a tree or deque.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Expand right to grow the sum, then shrink left while the sum stays at least target, tracking the shortest window. Because values are positive, left only moves forward: O(n) time and O(1) space.