Minimum Size Subarray Sum
Signals to notice
Brute force first
Check every subarray sum. Each subarray independently summed. That direct path helps you understand the question, but it tends to treat every possibility as brand new instead of learning from earlier steps.
The key insight
Sliding window: expand right to accumulate sum ≥ target. Then shrink left to minimize length while maintaining sum ≥ target. Track the minimum length. Once you hold onto the right piece of information from moment to moment, the problem feels less like trial and error and more like following a shape that was there all along.
What must stay true
Because all numbers are positive, adding elements only increases the sum. Once the sum ≥ target, shrinking from the left finds the minimum window with that property. When you keep that truth intact, each local choice supports the larger solution instead of fighting it.
Easy way to go wrong
This only works with positive numbers — with negatives, shrinking from the left might increase the sum. For mixed numbers, you'd need a deque-based approach. When the code becomes mechanical before the idea is clear, small edge cases start breaking the whole story.