How do you find the intersection node of two linked lists?

Q: How do you find the intersection node of two linked lists?

**Target Audience:** Beginners preparing for coding interviews — you should know what a linked list node is; everything else is explained **Difficulty:** Easy **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 hiking trails start at different car parks, wind through the forest separately, and at some point join into one shared path that leads to the same waterfall. Two friends start at the two car parks at the same moment and walk at the same speed. Where do the trails merge? If the trails were the same length before the junction, the friends would simply arrive at the junction together. They are not, so one friend gets there first and walks on alone. Here is the tric

The problem

Question presented to candidate: "You get the heads of two singly linked lists. If they merge into one shared tail, return the first shared node; otherwise return None. The lists have no cycles and must not be changed. Can you do it in O(1) extra memory? Please write it in Python."

What a strong answer should cover:

Intersection means the same node object, not equal values; the shared part is a common tail (Y shape).
A hash-set baseline in O(m + n) time and O(m) space.
An O(1)-space answer: align by length difference, or the switch-heads two pointers that both walk a + c + b nodes.
Why the switch-heads loop ends with no intersection: both pointers become None on the same step.
Python specifics: compare with is, write the switch as pa = pa.next if pa else head_b, and remember that a plain class hashes by identity so set() holds nodes, not values.

Clarifying questions expected:

"Same node, or equal values?"
"Can the lists contain cycles, and may I modify them?"

Code / implementation expected: Yes — get_intersection(head_a, head_b) in Python, O(1) space.

linked-list
two-pointers

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Solution

Target Audience: Beginners preparing for coding interviews — you should know what a linked list node is; everything else is explained Difficulty: Easy 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 hiking trails start at different car parks, wind through the forest separately, and at some point join into one shared path that leads to the same waterfall. Two friends start at the two car parks at the same moment and walk at the same speed. Where do the trails merge?

If the trails were the same length before the junction, the friends would simply arrive at the

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11 min read · 10 sections

Python code

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

PythonSwitch-heads intersection on Y-shaped and separate lists

Practice

PythonHash set, length difference and switch heads on every small shape

Practice

Discussion

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