Sort Intervals by Start
Order events before processing.
Sort Intervals by Start is a free Coding Interview Prep lesson on CoddyKit — lesson 1 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.
What an Interval Is
An interval is just a pair of numbers, a start and an end, like [2, 5]. Most interval problems are a list of these pairs. 📏
Order Brings Sanity
Raw intervals arrive in any order, which makes them hard to reason about. Sorting them first turns chaos into a clean left-to-right sweep.
Sort by Start
The default move is to sort by the start value. Now each interval begins at or after the one before it, so you can scan forward once.
intervals.sort(key=lambda x: x[0])Tuples Sort Naturally
If you store intervals as tuples, Python sorts by the first element, then the second, for free. No key function is even needed here.
intervals = [(3, 7), (1, 4), (2, 5)]
intervals.sort()Why Start First
Sorting by start lets you process events in time order. The next interval can only begin later, which is the key invariant for sweeping.
Ties on Start
When two intervals share a start, the secondary key decides their order. Sorting by (start, end) keeps shorter ones first, which often helps.
intervals.sort(key=lambda x: (x[0], x[1]))Sometimes Sort by End
A few problems, like scheduling the most events, sort by end instead. Pick the key that matches what your sweep needs to know.
intervals.sort(key=lambda x: x[1])The Cost of Sorting
Sorting takes O(n log n) time, which is cheap and usually dominates these problems. The sweep that follows is only O(n).
Keep Extra Data Attached
If each interval carries an id or weight, sort the whole record, not just the bounds. The key controls order while the data rides along.
intervals.sort(key=lambda iv: iv[0]) # iv = (start, end, id)Sort Then Sweep
Almost every interval algorithm is sort first, then sweep. Get the order right and merging, counting, and scheduling become simple loops.
A Quick Mental Model
Picture the intervals as guests arriving at a party. Sorting by start lines them up by arrival time so you can greet them one by one.
Quick Check
You are about to merge a list of intervals.
Recap
An interval is a start-end pair, and sorting by start turns a messy list into a clean sweep. Sort first, then process forward in O(n). 🚀
Frequently asked questions
Is the “Sort Intervals by Start” lesson free?
Yes — the full text of “Sort Intervals by Start” 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 “Sort Intervals by Start”?
Order events before processing. 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 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Sort Intervals by Start” 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
- Sort Intervals by Start
- Merge Overlapping Intervals
- Line Sweep for Max Overlap
- Minimum Removals for No Overlap