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feat: add Middle of the Linked List
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‎.templates/leetcode/json/middle_of_the_linked_list.json‎

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{
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"problem_name": "middle_of_the_linked_list",
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"solution_class_name": "Solution",
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"problem_number": "876",
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"problem_title": "Middle of the Linked List",
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"difficulty": "Easy",
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"topics": "Linked List, Two Pointers",
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"tags": ["grind-75"],
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"readme_description": "Given the `head` of a singly linked list, return *the middle node of the linked list*.\n\nIf there are two middle nodes, return **the second middle** node.",
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"readme_examples": [
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{
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"content": "![Example 1](https://assets.leetcode.com/uploads/2021/07/23/lc-midlist1.jpg)\n\n```\nInput: head = [1,2,3,4,5]\nOutput: [3,4,5]\n```\n**Explanation:** The middle node of the list is node 3."
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},
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{
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"content": "![Example 2](https://assets.leetcode.com/uploads/2021/07/23/lc-midlist2.jpg)\n\n```\nInput: head = [1,2,3,4,5,6]\nOutput: [4,5,6]\n```\n**Explanation:** Since the list has two middle nodes with values 3 and 4, we return the second one."
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}
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],
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"readme_constraints": "- The number of nodes in the list is in the range `[1, 100]`.\n- `1 <= Node.val <= 100`",
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"readme_additional": "",
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"solution_imports": "from leetcode_py import ListNode",
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"solution_methods": [
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{
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"name": "middle_node",
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"parameters": "head: ListNode | None",
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"return_type": "ListNode | None",
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"dummy_return": "None"
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}
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],
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"test_imports": "import pytest\nfrom leetcode_py.test_utils import logged_test\nfrom leetcode_py import ListNode\nfrom .solution import Solution",
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"test_class_name": "MiddleOfTheLinkedList",
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"test_helper_methods": [
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{ "name": "setup_method", "parameters": "", "body": "self.solution = Solution()" }
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],
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"test_methods": [
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{
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"name": "test_middle_node",
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"parametrize": "head_list, expected_list",
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"parametrize_typed": "head_list: list[int], expected_list: list[int]",
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"test_cases": "[([1, 2, 3, 4, 5], [3, 4, 5]), ([1, 2, 3, 4, 5, 6], [4, 5, 6]), ([1], [1]), ([1, 2], [2])]",
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"body": "head = ListNode.from_list(head_list)\nexpected = ListNode.from_list(expected_list)\nresult = self.solution.middle_node(head)\nassert result == expected"
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}
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],
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"playground_imports": "from solution import Solution\nfrom leetcode_py import ListNode",
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"playground_test_case": "# Example test case\nhead_list = [1, 2, 3, 4, 5]\nexpected_list = [3, 4, 5]",
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"playground_execution": "head = ListNode.from_list(head_list)\nresult = Solution().middle_node(head)\nresult",
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"playground_assertion": "expected = ListNode.from_list(expected_list)\nassert result == expected"
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}

‎Makefile‎

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PYTHON_VERSION = 3.13
2-
PROBLEM ?= diameter_of_binary_tree
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PROBLEM ?= middle_of_the_linked_list
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FORCE ?= 0
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COMMA := ,
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‎leetcode/middle_of_the_linked_list/README.md‎

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# Middle of the Linked List
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**Difficulty:** Easy
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**Topics:** Linked List, Two Pointers
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**Tags:** grind-75
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**LeetCode:** [Problem 876](https://leetcode.com/problems/middle-of-the-linked-list/description/)
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## Problem Description
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Given the `head` of a singly linked list, return _the middle node of the linked list_.
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If there are two middle nodes, return **the second middle** node.
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## Examples
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### Example 1:
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![Example 1](https://assets.leetcode.com/uploads/2021/07/23/lc-midlist1.jpg)
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```
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Input: head = [1,2,3,4,5]
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Output: [3,4,5]
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```
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**Explanation:** The middle node of the list is node 3.
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### Example 2:
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![Example 2](https://assets.leetcode.com/uploads/2021/07/23/lc-midlist2.jpg)
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```
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Input: head = [1,2,3,4,5,6]
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Output: [4,5,6]
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```
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**Explanation:** Since the list has two middle nodes with values 3 and 4, we return the second one.
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## Constraints
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- The number of nodes in the list is in the range `[1, 100]`.
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- `1 <= Node.val <= 100`

‎leetcode/middle_of_the_linked_list/__init__.py‎

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‎leetcode/middle_of_the_linked_list/playground.ipynb‎

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{
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"cells": [
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{
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"cell_type": "code",
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"execution_count": 5,
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"id": "imports",
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"metadata": {},
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"outputs": [],
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"source": [
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"from solution import Solution\n",
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"\n",
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"from leetcode_py import ListNode"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 6,
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"id": "setup",
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"metadata": {},
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"outputs": [],
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"source": [
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"# Example test case\n",
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"head_list = [1, 2, 3, 4, 5]\n",
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"expected_list = [3, 4, 5]"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 7,
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"id": "execute",
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/html": [
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"<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>\n",
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"<!DOCTYPE svg PUBLIC \"-//W3C//DTD SVG 1.1//EN\"\n",
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" \"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd\">\n",
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"<!-- Generated by graphviz version 13.1.2 (20250808.2320)\n",
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" -->\n",
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"<!-- Pages: 1 -->\n",
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"<svg width=\"242pt\" height=\"44pt\"\n",
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" viewBox=\"0.00 0.00 242.00 44.00\" xmlns=\"http://www.w3.org/2000/svg\" xmlns:xlink=\"http://www.w3.org/1999/xlink\">\n",
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"<g id=\"graph0\" class=\"graph\" transform=\"scale(1 1) rotate(0) translate(4 40)\">\n",
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"<polygon fill=\"white\" stroke=\"none\" points=\"-4,4 -4,-40 238,-40 238,4 -4,4\"/>\n",
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"<!-- node_0 -->\n",
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"<g id=\"node1\" class=\"node\">\n",
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"<title>node_0</title>\n",
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"<path fill=\"lightblue\" stroke=\"black\" d=\"M42,-36C42,-36 12,-36 12,-36 6,-36 0,-30 0,-24 0,-24 0,-12 0,-12 0,-6 6,0 12,0 12,0 42,0 42,0 48,0 54,-6 54,-12 54,-12 54,-24 54,-24 54,-30 48,-36 42,-36\"/>\n",
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"<text xml:space=\"preserve\" text-anchor=\"middle\" x=\"27\" y=\"-12.95\" font-family=\"Times,serif\" font-size=\"14.00\">3</text>\n",
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"</g>\n",
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"<!-- node_1 -->\n",
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"<g id=\"node2\" class=\"node\">\n",
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"<title>node_1</title>\n",
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"<path fill=\"lightblue\" stroke=\"black\" d=\"M132,-36C132,-36 102,-36 102,-36 96,-36 90,-30 90,-24 90,-24 90,-12 90,-12 90,-6 96,0 102,0 102,0 132,0 132,0 138,0 144,-6 144,-12 144,-12 144,-24 144,-24 144,-30 138,-36 132,-36\"/>\n",
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"<text xml:space=\"preserve\" text-anchor=\"middle\" x=\"117\" y=\"-12.95\" font-family=\"Times,serif\" font-size=\"14.00\">4</text>\n",
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"</g>\n",
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"<!-- node_0&#45;&gt;node_1 -->\n",
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"<g id=\"edge1\" class=\"edge\">\n",
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"<title>node_0&#45;&gt;node_1</title>\n",
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"<path fill=\"none\" stroke=\"black\" d=\"M54.4,-18C61.89,-18 70.18,-18 78.2,-18\"/>\n",
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"<polygon fill=\"black\" stroke=\"black\" points=\"78.1,-21.5 88.1,-18 78.1,-14.5 78.1,-21.5\"/>\n",
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"</g>\n",
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"<!-- node_2 -->\n",
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"<g id=\"node3\" class=\"node\">\n",
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"<title>node_2</title>\n",
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"<path fill=\"lightblue\" stroke=\"black\" d=\"M222,-36C222,-36 192,-36 192,-36 186,-36 180,-30 180,-24 180,-24 180,-12 180,-12 180,-6 186,0 192,0 192,0 222,0 222,0 228,0 234,-6 234,-12 234,-12 234,-24 234,-24 234,-30 228,-36 222,-36\"/>\n",
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"<text xml:space=\"preserve\" text-anchor=\"middle\" x=\"207\" y=\"-12.95\" font-family=\"Times,serif\" font-size=\"14.00\">5</text>\n",
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"</g>\n",
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"<!-- node_1&#45;&gt;node_2 -->\n",
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"<g id=\"edge2\" class=\"edge\">\n",
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"<title>node_1&#45;&gt;node_2</title>\n",
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"<path fill=\"none\" stroke=\"black\" d=\"M144.4,-18C151.89,-18 160.18,-18 168.2,-18\"/>\n",
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"<polygon fill=\"black\" stroke=\"black\" points=\"168.1,-21.5 178.1,-18 168.1,-14.5 168.1,-21.5\"/>\n",
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"</g>\n",
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"</g>\n",
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"</svg>\n"
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],
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"text/plain": [
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"ListNode([3, 4, 5])"
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]
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},
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"execution_count": 7,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"head = ListNode.from_list(head_list)\n",
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"result = Solution().middle_node(head)\n",
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"result"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 8,
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"id": "test",
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"metadata": {},
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"outputs": [],
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"source": [
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"expected = ListNode.from_list(expected_list)\n",
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"assert result == expected"
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "leetcode-py-py3.13",
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"language": "python",
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"name": "python3"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 3
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python3",
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"pygments_lexer": "ipython3",
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"version": "3.13.7"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 5
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}

‎leetcode/middle_of_the_linked_list/solution.py‎

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from leetcode_py import ListNode
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class Solution:
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# Time: O(n)
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# Space: O(1)
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def middle_node(self, head: ListNode | None) -> ListNode | None:
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slow = fast = head
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while fast and fast.next:
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assert slow is not None
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slow = slow.next
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fast = fast.next.next
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return slow

‎leetcode/middle_of_the_linked_list/tests.py‎

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import pytest
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from leetcode_py import ListNode
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from leetcode_py.test_utils import logged_test
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from .solution import Solution
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class TestMiddleOfTheLinkedList:
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def setup_method(self):
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self.solution = Solution()
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@pytest.mark.parametrize(
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"head_list, expected_list",
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[
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([1, 2, 3, 4, 5], [3, 4, 5]), # odd length
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([1, 2, 3, 4, 5, 6], [4, 5, 6]), # even length
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([1], [1]), # single node
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([1, 2], [2]), # two nodes
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([1, 2, 3], [2, 3]), # three nodes
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([1, 2, 3, 4], [3, 4]), # four nodes
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([10, 20, 30, 40, 50, 60, 70], [40, 50, 60, 70]), # larger odd
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([5, 15, 25, 35], [25, 35]), # larger even
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],
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)
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@logged_test
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def test_middle_node(self, head_list: list[int], expected_list: list[int]):
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head = ListNode.from_list(head_list)
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expected = ListNode.from_list(expected_list)
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result = self.solution.middle_node(head)
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assert result == expected

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