How do you merge overlapping intervals? | Interview Question

Q: How do you merge overlapping intervals?

**Target Audience:** Beginners preparing for coding interviews — you need loops, lists/arrays and sorting in Python. **Difficulty:** Medium **Language:** Python 3 > **How to read this doc:** every idea is explained in plain words first. When a word appears that an interviewer will expect you to know, it is tagged `📌 Interview term:` with a one-line definition, and every tagged term is collected in the Quick Glossary at the end. Every program on this page was actually run, and the output you see in the **Verified** section is copied from that run. ## 1. Why This Even Matters — A Story First Your calendar app shows that you are busy 9–11, 10–12, and 14–15, and your manager asks a simple question: "When are you actually busy?" You would not answer with three blocks — you would say "9 to 12, and 2 to 3 in the afternoon". You merged the first two because they overlap. That is the whole problem: given a pile of `[start, end]` ranges in any order, squash every group of overlapping ra

The problem

Question presented to candidate: "You are given a list of intervals, each a list [start, end], in no particular order. Write a Python function that merges all overlapping intervals and returns the merged list sorted by start."

What a strong answer should cover:

Sort by start, then one pass that either extends the last merged interval or starts a new one.
Use max(last_end, end) when extending, because an interval can be nested inside the previous one.
Complexity: O(n log n) for the sort plus O(n) for the pass; O(n) for the output.
Touching intervals ([1, 4] and [4, 5]): <= merges them; confirm the expected behaviour.
Python details: sorted(..., key=lambda iv: iv[0]) returns a new list while list.sort() mutates the caller's list; append a new [start, end] so mutating merged[-1] never changes the input.

Clarifying questions expected:

Do touching intervals count as overlapping?
May I modify the input list?

Code / implementation expected: Yes — a function merge(intervals) -> list[list[int]] in Python 3.

intervals
sorting

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for coding interviews — you need loops, lists/arrays and sorting in Python.
Difficulty: Medium
Language: Python 3

How to read this doc: every idea is explained in plain words first. When a word appears that an interviewer will expect you to know, it is tagged 📌 Interview term: with a one-line definition, and every tagged term is collected in the Quick Glossary at the end. Every program on this page was actually run, and the output you see in the Verified section is copied from that run.

1. Why This Even Matters — A Story First

Your calendar app shows that you are busy 9–11, 10–12, and 14–15, and your manager asks a simple question: "When are you actually busy?" You would not answer with three blocks — you would say "9 to 12, and 2 to 3 in the afternoon". You merged the first two because they overlap.

That is the who

The solution is hidden so you can try first

4 min read · 11 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonSort + one pass on seven inputs (touching, nested, unsorted, empty)

Practice

PythonThe three classic bugs next to the correct merge

Practice

Discussion

Other approaches, follow-ups and how it went in real interviews.

Sign in to join the discussion.

No comments yet. Share how you would answer it.

The solution is hidden so you can try first

Keep your momentum going.