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This week, we'll solve a problem that I was asked in an Apple interview and I could not solve it. Let's see if you can.

Today we will cover:

• Basic Calculator

• Rate Limiting Algorithms

Read time: under 4 minutes

CODING CHALLENGE

Basic Calculator

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Example 1:

Input: s = " 2-1 + 2 "
Output: 3

Example 2:

Input: s = "(1+(4+5+2)-3)+(6+8)"
Output: 23

Solve the problem here before reading the solution.

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SOLUTION

We can solve this problem using a stack to handle the parentheses. The main idea is to keep track of our current result and use the stack to remember previous results when we go into parentheses.

We'll use three variables: result to store our running total, number to build up multi-digit numbers, and sign to remember if the next number should be added or subtracted.

As we go through each character, we build numbers digit by digit and handle the + and - operators by updating our result and sign.

When we see an opening parenthesis (, we save our current result and sign on the stack, then start fresh for the expression inside the parentheses. When we see a closing parenthesis ), we finish calculating the expression inside, then pop our saved values from the stack to combine everything together.

This works because the stack helps us handle nested parentheses correctly. Each time we go deeper into parentheses, we save where we were and start over. When we come back out, we pick up where we left off.

The time complexity is O(n) because we look at each character once. The space complexity is O(n) for the stack, which grows with the number of nested parentheses.

SYSTEM DESIGN

Rate Limiting Algorithms

When building popular web services, you need to protect them from being overwhelmed by too many requests. Rate limiting helps by controlling how many requests a client can make within a specific time period. Let's look at some common algorithms used for rate limiting.

The simplest approach is Fixed Window rate limiting. Imagine dividing time into fixed windows (like minutes) and allowing only N requests per window. When a new minute starts, the counter resets. While simple to implement, this can lead to burst traffic at window boundaries. If your limit is 100 requests per minute, a client could make 100 requests at 11:59 and another 100 at 12:00, sending 200 requests in just two seconds.

Sliding Window rate limiting solves this boundary problem. Instead of fixed windows, it uses a rolling time window. When a request arrives, we count all requests in the last minute (or whatever time period we choose). This prevents burst traffic at window boundaries but requires more memory since we need to track the timestamp of each request.

Token Bucket is another popular algorithm. Imagine a bucket that holds tokens. New tokens are added at a fixed rate up to the bucket's capacity. Each request needs one token. If there are no tokens, the request is rejected. This allows for bursts of traffic (up to the bucket size) while maintaining a long-term rate limit.

Leaky Bucket works similarly but focuses on smoothing out traffic. Requests enter a queue (the bucket) that processes them at a constant rate. If the bucket is full, new requests are rejected. This ensures steady outflow but can lead to delays since requests wait in the queue.

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NEWS

This Week in the Tech World

Bumble cuts 30% of workforce: Dating app Bumble is laying off 240 employees to save $40 million annually. It plans to reinvest this money in product development. The company raised its Q2 revenue forecast and saw shares jump 20% after the announcement.

Amazon launches more Kuiper satellites: Amazon sent 27 more internet satellites into orbit, competing with Starlink. The company needs 1,618 satellites by July 2026 to meet FCC requirements.

US House bans WhatsApp: House staffers can't use WhatsApp on government devices due to privacy concerns. Meta strongly disagrees, saying WhatsApp offers better security than most approved apps.

Meta tried buying Runway AI: Meta discussed acquiring AI video startup Runway before its $14.3 billion Scale AI investment. The takeover talks dissolved early, but Meta continues aggressive AI talent hunting.

DeepSeek aids China's military: US officials say Chinese AI firm DeepSeek supports military operations and evaded chip export controls using Southeast Asian shell companies to access restricted semiconductors.

Google faces UK search changes: Britain's competition watchdog may force Google to add choice screens and fair search rankings. The move targets Google's search dominance under new UK competition rules.

Tesla signs China battery deal: Tesla inked its first deal to build China's largest grid-scale battery plant worth $556 million. The project helps balance China's electricity grid amid US-China trade tensions.

Uber and Waymo expand to Atlanta: Waymo's self-driving cars are now available on Uber in Atlanta, covering 65 square miles. Riders pay regular Uber rates with no tipping required.

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Cheers,

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