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Everyone knows how to reverse a LinkedList. But this week's challenge adds a twist that changes everything.

Today we will cover:

• Reverse Nodes in k-Group

• Authentication Methodologies

Read time: under 4 minutes

CODING CHALLENGE

Reverse Nodes in k-Group

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes, in the end, should remain as it is.

You may not alter the values in the list's nodes, only nodes themselves may be changed.

Example 1:

Input: head = [1,2,3,4,5], k = 2
Output: [2,1,4,3,5]

Example 2:

Input: head = [1,2,3,4,5], k = 3
Output: [3,2,1,4,5]

Solve the problem here before reading the solution.

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SOLUTION

To solve this problem, we'll use an iterative approach that reverses groups of k nodes while maintaining the overall structure of the list.

We'll use a helper function to reverse a portion of the linked list and keep track of the groups of k nodes. If the remaining nodes are less than k, we'll leave them as they are.

Here is how the solution works:

1. We use a dummy node to handle changes in the list's head

2. We check if we have k nodes to reverse

3. We use a separate helper function reverse to reverse a portion of the list

4. We keep track of the previous group, current group, and next group

5. After reversing, we reconnect the groups

6. If there are fewer than k nodes remaining, we leave them as they are

The time complexity of this solution is O(n), where n is the number of nodes in the linked list.

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SYSTEM DESIGN

Authentication Methodologies

Building secure applications requires robust user authentication. Let's look at different ways to authenticate users and understand when to use each method.

Password-based authentication is the most common method. Users provide a username and password which is checked against stored credentials. While simple to implement, this method has limitations. Users often choose weak passwords or reuse them across sites. To make password authentication more secure, you can add requirements for password complexity and implement rate limiting to prevent brute force attacks.

Two-Factor Authentication (2FA) adds an extra layer of security. After entering their password, users must provide a second form of verification. This could be a code sent via SMS or generated by an authenticator app. 2FA significantly improves security because even if attackers steal a password, they can't access the account without the second factor.

Token-based authentication uses temporary access tokens instead of sending credentials with every request. When users log in, they receive a token (usually a JWT) that they include in subsequent requests. Tokens can contain user information and permissions, and they automatically expire after a set time. This makes token-based auth ideal for modern web applications and APIs.

OAuth is used for authorizing applications to access user data on other platforms. For example, when you "Login with Google," you're using OAuth. The application gets a token to access your Google data without ever seeing your Google password. OAuth is perfect when you want to integrate with other services or provide "Sign in with X" functionality.

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NEWS

This Week in the Tech World

OpenAI Sora 2 Released: OpenAI launched Sora 2, the next generation of its text-to-video AI model with improved video generation capabilities. The model allows users to create realistic videos from text prompts with better quality and control than its predecessor.

Claude Sonnet 4.5 Debuts: Anthropic released Claude Sonnet 4.5, claiming it's the best coding model available. The model can work autonomously for 30+ hours, scoring 77.2% on SWE-bench software engineering tests and outperforming GPT-5.

DeepSeek Releases V3.2-Exp: Chinese AI company DeepSeek unveiled V3.2-Exp, an experimental model with improved efficiency and 50% lower API costs. The release follows DeepSeek's Nature paper revealing R1 training cost just $294,000, challenging assumptions about expensive AI development.

npm Worm Compromises 500+ Packages: Self-replicating malware dubbed "Shai-Hulud" infected 500+ npm packages including @ctrl/tinycolor and CrowdStrike libraries. The worm steals credentials and automatically spreads to other packages. GitHub responded by blocking uploads and removing compromised packages.

PostgreSQL 18 Ships: PostgreSQL 18 was released, featuring a new asynchronous I/O subsystem delivering up to 3x performance improvements. The release includes better indexing, faster major-version upgrades, virtual generated columns, and OAuth 2.0 authentication support for easier SSO integration.

DoorDash unveils delivery robot Dot: Food delivery giant launches autonomous robot reaching 20 mph carrying 30 pounds for Phoenix merchants. Bot uses eight cameras and lidar sensors while SmartScale feature cuts missing item complaints by 30%.

Microsoft touts "vibe working" for Office: Company adds AI agent mode to Excel and Word letting users describe desired outputs iteratively. Copilot manages 57% accuracy versus human's 71% on spreadsheets, requiring careful auditing.

BONUS

Just for laughs 😏

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Cheers,

Sahil + Sarra