What is the two-pointer technique? | Interview Question

Q: What is the two-pointer technique?

**Target Audience:** Beginners preparing for coding interviews — no prior algorithms knowledge assumed **Difficulty:** Medium **Language:** Python 3 > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. All code is Python, and every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First Two friends are looking for two books on a long, alphabetised shelf whose page counts add up to exactly 500, so the pair fits a 500-page reading challenge. The slow way: pick the first book, compare it with every other book, then pick the second book, compare it with every other book, and so on. With 1,000 books that is about half a million comparisons. The clever way: the shelf is sorted by page count. One friend stands at the thinnest book, the other at the thickest. They add the two page counts.

The problem

Question presented to candidate: "Explain the two-pointer technique. Then, in Python, use it to find two numbers in a sorted list that add up to a target, and tell me how it beats the obvious nested loop."

What a strong answer should cover:

Two indices moving through the data under a rule; the two shapes (converging from both ends, and same-direction read/write).
Why moving a pointer is safe: the sorted (monotonic) order lets one comparison rule out a whole element.
O(n) time and O(1) extra space versus O(n²) for checking every pair; sorting first adds O(n log n).
When a hash map is the better tool (unsorted input, original indices needed).
Python specifics: tuple swap a[i], a[j] = a[j], a[i], len(arr) - 1 is -1 for an empty list so the loop simply does not run, and avoiding slices like arr[1:] inside the loop because each one copies.

Clarifying questions expected:

"Is the list sorted, and can it contain duplicates or negative numbers?"
"Should I return indices or values, and what if no pair exists?"

Code / implementation expected: Yes — a Python pair_sum(arr, target) with left/right indices, plus a same-direction example such as removing duplicates in place.

two-pointers

Try it yourself

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

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

Solution

Target Audience: Beginners preparing for coding interviews — no prior algorithms knowledge assumed Difficulty: Medium Language: Python 3

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. All code is Python, and every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

Two friends are looking for two books on a long, alphabetised shelf whose page counts add up to exactly 500, so the pair fits a 500-page reading challenge. The slow way: pick the first book, compare it with every other book, then pick the second book, compare it with every other book, and so on. With 1,000 books that is about half a million comparisons.

The clever way: the shelf is sorted by page cou

The solution is hidden so you can try first

8 min read · 9 sections

Python code

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

PythonPair sum with converging pointers, and a count of checks vs brute force

Practice

PythonSame-direction pointers: remove duplicates and reverse in place

Practice

Discussion

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