Variable Window with Two Pointers
Grow and shrink to meet a condition.
Variable Window with Two Pointers is a free Coding Interview Prep lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Coding Interview Prep learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
When the Window Breathes
Some problems do not fix the window length. Instead the window grows and shrinks to keep a condition true, like a sum staying under a limit.
Two Pointers, One Window
Keep two indices, left and right, marking the window edges. The right pointer expands while left trails behind to shrink when needed.
left = 0
window = 0Expand to the Right
Walk right across every element and include it in the window. Update your running state, such as adding the new value to a sum.
for right in range(n):
window += a[right]Shrink When You Must
While the window violates the rule, move left rightward and remove that element. This restores the condition without going back.
while window > limit:
window -= a[left]
left += 1Read the Valid Window
Once the inner loop ends, the window from left to right is valid. Its length is right minus left plus one, ready to use.
length = right - left + 1Record Your Best
Update your answer with this valid window, often the longest one seen. Do this every iteration so nothing is missed.
best = max(best, right - left + 1)Why It Is Linear
Each pointer only moves forward, never back. left and right together advance at most n steps, so the whole sweep is O(n).
The Monotonic Requirement
This works when extending the window only makes the condition harder to satisfy. That monotonic behavior is what lets left never rewind.
Longest vs Shortest
For the shortest valid window, shrink while the rule still holds and record before breaking. The pointer mechanics stay the same.
while window >= target:
best = min(best, right - left + 1)
window -= a[left]
left += 1Beware Empty Windows
If shrinking can empty the window, guard against left passing right. Also confirm an answer was actually found before returning it.
Spotting the Pattern
Reach for a variable window when a problem asks for the longest or shortest contiguous stretch meeting a condition on its elements.
Quick Check
You run a two-pointer variable window over an array of size n.
Recap
Expand right to include elements, shrink left while the rule breaks, and record each valid window. Forward-only pointers keep it O(n). ✅
Frequently asked questions
Is the “Variable Window with Two Pointers” lesson free?
Yes — the full text of “Variable Window with Two Pointers” is free to read here on the web, and the Coding Interview Prep course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Coding Interview Prep course, upgrade to CoddyKit PRO.
What will I learn in “Variable Window with Two Pointers”?
Grow and shrink to meet a condition. You practise Coding Interview Prep with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Coding Interview Prep?
No prior experience is required. Coding Interview Prep on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Variable Window with Two Pointers” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Coding Interview Prep lesson?
Yes. Every Coding Interview Prep lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Fixed-Size Window Sums
- Variable Window with Two Pointers
- Longest Substring Without Repeats
- Count Windows That Satisfy a Rule