The Intervals Pattern
Intervals are sorted by one endpoint, then swept in order while tracking what is currently open. Almost every interval problem reduces to sorting correctly and then handling one comparison between the current interval and the previous one.
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Interactive problems2
Free to openHow to recognise a intervals problem
The input is a list of start/end pairs — meetings, bookings, ranges — and the question involves merging, overlap, insertion or counting concurrency. If the input looks like [[1,3],[2,6],[8,10]], this is the pattern.
Time and space complexity
O(n log n), dominated by the sort; the sweep itself is O(n). Choosing the wrong sort key is the most common failure, and it is usually end time rather than start time.
Intervals practice problems
Each problem pairs a worked explanation with an interactive visualizer you step through yourself, plus the implementation in JavaScript, Python, Java and C++.
Related patterns and concepts
Intervals is one of the 16 coding interview patterns, and its problems also appear in the DSA 75 interview sprint.