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Motmans & Partners Interview Questions

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Data Structures & Algorithms

Write a function n_frequent_words that returns the top N frequent words and their frequencies, and state its run-time

Get Top N Frequent Words

Given an array and a target integer, write a function that returns the indices of two integers in the array that add up to the target integer.

Target Indices

Write a function `sort_lists` to combine sorted integer lists into one sorted list without using libraries or `sort`/`sorted`

Merge N Sorted Lists

Implementing a priority queue used linked lists.

Priority Queue Using Linked List

Write a function that tests whether a string of brackets is balanced.

The Brackets Problem

Product Sense & Metrics

How would you measure the success of an online marketplace introducing an audio chat feature given a dataset of their usage?

Audio Chat Success

Google Maps Improvement

What are the most important metrics for WhatsApp

WhatsApp Metrics

How would you approach sizing the market, segmenting users, identifying competitors, and building a marketing plan for a new smart fitness tracker?

Smart Fitness Tracker Launch

Data Pipelines

How would you build an ETL pipeline to ingest Stripe payment data into a warehouse for revenue dashboards and analytics

Payment Data Pipeline

Redesign batch credit card processing to enable real-time streaming for fraud detection and reporting.

Real-Time Transaction Streaming

How would you design data pipelines to handle rapid growth in data volume while maintaining reliability?

Scalable Data Pipelines

Machine Learning

How would you justify the complexity of building such a model and explain the predictions to non-technical stakeholders

Justify a Neural Network

Design the YouTube video recommendation system and explain important factors to keep in mind

Youtube Recommendations

How would you approach and build a hot posts or trending posts sort feature to increase engagement

Trending Sort

Behavioral

Describing a data project and its challenges

Hurdles In Data Projects

How would you negotiate and resolve disagreements when a client rejects your proposed solution?

Client Solution Pushback

<ol>
<li>Given an example paragraph string and an integer <code>N</code>, write a function <code>n_frequent_words</code> that returns the top <code>N</code> frequent words in the posting and the frequencies for each word.</li>

<li>What’s the function run-time?</li>
</ol>

Example:

Input:

<pre tabindex="0" class="chroma"><code>posting = &#34;&#34;&#34;
Herbal sauna uses the healing properties of herbs in combination with distilled water. 
The water evaporates and distributes the effect of the herbs throughout the room. 
A visit to the herbal sauna can cause real miracles, especially for colds. 
&#34;&#34;&#34; 
n = 3
</code></pre>

Output:

<pre tabindex="0" class="chroma"><code>n_frequent_words(posting,N) = [
 (&#39;the&#39;, 6), 
 (&#39;herbal&#39;, 2), 
 (&#39;sauna&#39;, 2),
]
</code></pre>

You’re given a string that may contain the characters <code>{</code>, <code>}</code>, <code>[</code>, <code>]</code>, <code>(</code>, and <code>)</code>.

Task: Verify that the string is balanced. A balanced string is one where every opening character, <code>{</code>, <code>[</code>, or <code>(</code>, has a corresponding closing character, <code>}</code>, <code>]</code>, or <code>)</code>.

Write a function called <code>is_balanced(string: str) -&gt; bool</code> which verifies the balance of a string.

Example:

<pre tabindex="0" class="chroma"><code>is_balanced(&#39;(())[]{}&#39;) -&gt; True
</code></pre>

<pre tabindex="0" class="chroma"><code>is_balanced(&#39;{([(){}])()}&#39;) -&gt; True
</code></pre>

<pre tabindex="0" class="chroma"><code>is_balanced(&#39;{}[]())&#39;) -&gt; False
</code></pre>

<hr/>

Let’s say that you’re the PM on Google Maps.

<ol>
<li>How would you improve Google Maps?</li>

<li>What metrics would you check to see if your feature improvements are successful?</li>
</ol>

You work as a data engineer at a global financial services company. The current pipeline ingests millions of daily credit card transactions via nightly batch jobs. The business now requires real-time processing and analysis for each transaction to support instant fraud detection and real-time reporting.

How would you redesign the ingestion and processing architecture to enable reliable, scalable, and consistent real-time streaming of transaction data?

Draft a design proposal outlining your approach.

Given a <code>ListNode</code> representing the head of a linked list, write a function <code>is_cyclic</code> which returns <code>True</code> if a linked list is cyclic and <code>False</code> if it is not. A linked list is cyclic when there is no tail to the linked list, and the supposed tail is attached to another node inside the list itself, creating a cycle.

A <code>ListNode</code> is defined as:

<pre tabindex="0" class="chroma"><code>class ListNode:
	value: ListNode = None
 next: ListNode = None
</code></pre>

Example:

Input:

<img src="/s3/questions/1a65651d-a483-4481-b073-2738d65ab9d7.png" alt="image"/>

Output:

<pre tabindex="0" class="chroma"><code>def is_cyclic(head) -&gt; True
</code></pre>

Cyclic Detection

Given an array and a target integer, write a function <code>sum_pair_indices</code> that returns the indices of two integers in the array that add up to the target integer. If not found, just return an empty list.

Note: Can you do it on \(O(n)\) time?

Note: Even though there could be many solutions, only one needs to be returned.

Example 1:

Input:

<pre tabindex="0" class="chroma"><code>array = [1 2 3 4] 
target = 5 
</code></pre>

Output:

<pre tabindex="0" class="chroma"><code>def sum_pair_indices(array, target) -&gt; [0 3] or [1 2]
</code></pre>

Example 2:

Input:

<pre tabindex="0" class="chroma"><code>array = [3]
target = 6 
</code></pre>

Output:

<pre tabindex="0" class="chroma"><code>Do NOT return [0 0] as you can&#39;t use an index twice.
</code></pre>

Tell me about a data project that didn’t go the way you expected. What did you set out to do, what surprised you, and how did you handle it?

Priority queues are an abstract data structure responsible for allowing enqueuing items with an attached priority. While typically implemented with a heap, implement a priority queue using a linked list.

The Priority Queue implementation should support the following operations:

<ul>
<li><code>insert</code>(element, priority): This operation should be able to insert an element into the Priority Queue, along with its corresponding priority.</li>
<li><code>delete()</code>: This operation should remove and return the element with the highest priority. If multiple elements share the same highest priority, the element first enqueued should be returned. In the case that the queue is empty, return None.</li>
<li><code>peek()</code>: This operation should return the element with the highest priority without removing it from the Priority Queue. Again, in the case of equal highest priorities, the element first enqueued should be returned. In the case that the queue is empty, return None.</li>
</ul>

Note: Smaller priority values imply that they have higher priority.

<h3>Input</h3>

<pre tabindex="0" class="chroma"><code>queue.insert(2, 1)
queue.insert(3, 3)
</code></pre>

<h3>Output:</h3>

<pre tabindex="0" class="chroma"><code>[2,3]
</code></pre>

Given a list of sorted integer lists, write a function <code>sort_lists</code> to create a combined list while maintaining sorted order without importing any libraries or using the <code>&#39;sort&#39;</code> or <code>&#39;sorted&#39;</code> functions in Python.

Example:

Input:

<pre tabindex="0" class="chroma"><code>lists = [
[1,2,3,4,5,6],
[2,5,7,8],
[3,9,10,12],
[0,1,2,8]
]
</code></pre>

Output:

<pre tabindex="0" class="chroma"><code>def sort_lists(lists) -&gt; [0,1,1,2,2,3,3,4,5,5,6,7,8,8,9,10,12]
</code></pre>

Given an array of integers <code>nums</code> and an integer <code>k</code>, return the total number of subarrays whose sum equals to <code>k</code>.

A subarray is a contiguous non-empty sequence of elements within an array.

<h3>Example 1</h3>

Input:

<pre tabindex="0" class="chroma"><code>nums = [1,1,1], k = 2
</code></pre>

Output:

<pre tabindex="0" class="chroma"><code>def subarraySum(nums, k) -&gt; 2
</code></pre>

<h3>Example 2</h3>

Input:

<pre tabindex="0" class="chroma"><code>nums = [1,2,3], k = 3
</code></pre>

Output:

<pre tabindex="0" class="chroma"><code>def subarraySum(nums, k) -&gt; 2
</code></pre>

<h3>Constraints:</h3>

<ul>
<li><code>1 &lt;= nums.length &lt;= 2 * 10^4</code></li>
<li><code>-1000 &lt;= nums[i] &lt;= 1000</code></li>
<li><code>-10^7 &lt;= k &lt;= 10^7</code></li>
</ul>

Count the number of contiguous subarrays whose sum equals k

Subarray Sum Equals K

An online marketplace company has introduced a new feature that allows potential buyers and sellers to conduct audio chats with each other prior to transacting.

Let’s say we have two tables that represent this data.

Example:

Input:

<code>chats</code> table

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>buyer_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>seller_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>call_length</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>call_connected</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<code>marketplace_purchases</code> table

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>buyer_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>seller_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>item_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>purchase_amount</code></td>
<td>FLOAT</td>
</tr>
</tbody>
</table>

<ol>
<li>How would you measure the success of this new feature?</li>
<li>Write a query that can represent if the feature is successful or not.</li>
</ol>

You work as a backend engineer at a growing messaging startup. The team is tasked with building a scalable messenger service that supports individual and group chats, message delivery receipts, and media attachments.

How would you architect the core messaging components using object-oriented principles?

Write a technical design document describing your approach.

Design a scalable messenger system using OOP principles and write a technical design document.

Messenger Service Design

Concurrent & Distributed Systems

Meta’s trust and safety team has been running an ML model that detects accounts posting prohibited content (adult content, illegal goods, scam ads, policy-restricted ads) and bans them. Currently, approximately 5% of all content on the platform is prohibited.

The ML team has developed a new model intended to replace the existing one. You don’t yet know whether the new model performs better or worse.

How would you determine if the new system is working? Walk through the stakeholders affected, the key metrics you’d track, and how you’d distinguish a well-performing model from a poorly-performing one.

Designing an ML system for unsafe content detection

Unsafe Content ML Design

Responsible AI & Security

Create an <code>Array</code> class simulating the functionality of fixed-size arrays. The array’s size is <code>6</code>. The array’s constructor is already defined:

<pre tabindex="0" class="chroma"><code>class Array:

 def __init__(self):
 pass # Dynamically added to avoid tampering
 self._count = 0
 self._array = [None, None, None, None, None, None]
 self._MAX_CAPACITY = 6
</code></pre>

There is no need to implement the constructor. In the code editor, <code>__init__</code> will contain <code>pass</code>. The constructor’s code will be added dynamically to avoid any changes.

The <code>Array</code> class should have the following methods:

<ul>
<li><code>__len__()</code>: Returns the length of the array. For example, if the array is <code>[1, 2, None, None, None, None, None]</code>, <code>__len__()</code> will return <code>2</code>.</li>

<li><code>__getitem__(index)</code>: Returns the element at the specified index. Raises an <code>IndexError</code> when the index is out of range.</li>

<li><code>emplace_back(element)</code>: Places an element at the back of the array. Raises an <code>ArrayFull</code> exception when the array is full (length &gt; 6).</li>

<li><code>emplace_front(element)</code>: Places an element at the front of the array. Raises an <code>ArrayFull</code> exception when the array is full (length &gt; 6).</li>

<li><code>emplace(element, index)</code>: Places an element at the specified index. Raises an <code>ArrayFull</code> exception when the array is full (length &gt; 6).</li>
</ul>

Note: <code>ArrayFull</code> is already implemented. To raise it, simply use <code>raise ArrayFull()</code>.

Implement the addition operations of fixed length arrays.

Fixed Length Arrays: Addition

You work as a backend engineer at a company deploying a large language model (LLM) as a service for multiple clients. The LLM must handle concurrent user requests while maintaining low latency, high throughput, and robust fault tolerance.

How would you design the serving infrastructure for this LLM to efficiently manage simultaneous requests from multiple users?

Design a system to handle simultaneous requests to a deployed LLM model ensuring scalability, low latency, and reliability.

Concurrent LLM Serving

Given a string, write a function to determine if it is palindrome or not.

Note: A palindrome is a word/string that is read the same way forward as it is backward, e.g. <code>&#39;reviver&#39;</code>, <code>&#39;madam&#39;</code>, <code>&#39;deified&#39;</code> and <code>&#39;civic&#39;</code> are all palindromes, while <code>&#39;tree&#39;</code>, <code>&#39;music&#39;</code> and <code>&#39;person&#39;</code> are not palindromes.

Example:

Input:

<pre tabindex="0" class="chroma"><code>word1 = &#34;tree&#34;
word2 = &#34;radar&#34;
</code></pre>

Output:

<pre tabindex="0" class="chroma"><code>def is_palindrome(word1) -&gt; False
def is_palindrome(word2) -&gt; True
</code></pre>

Given a string, write a function to determine if it is palindrome or not.

String Palindromes

Write a function <code>data_stream_median</code> to calculate the new median from a stream of ordered integers and the new element which is not yet inserted into the stream.

The median is the middle value in an ordered integer list. If the size of the list is even, there is no middle value, so the median is the mean of the two middle values.

Example:

<pre tabindex="0" class="chroma"><code>new_value = 2
stream = [1, 2, 3, 4, 5, 6]
def data_stream_median(new_value, stream): -&gt; 3
</code></pre>

Write a function data_stream_median to calculate the median from a stream of integers.

Data Stream Median

You are given the layout of a rectangular building with rooms forming a grid. Each room has four doors to the room to the north, east, south, and west where exactly one door is unlocked and the other three doors are locked. In each time step, you can move to an adjacent room via an unlocked door.

Your task is to determine the minimum number of time steps required to get from the northwest corner to the southeast corner of the building.

Note: If the path doesn’t lead you to exit return <code>-1 .</code>

The input is given as:

<ul>
<li>a non-empty 2d-array of letters <code>&#39;N&#39;</code>, <code>&#39;E&#39;</code>, <code>&#39;S&#39;</code>, <code>&#39;W&#39;</code> named ‘building’</li>
<li>‘building[0][0]’ represents the open door at the northwest corner.</li>
<li>The rows of this array are associated with the north-south direction.</li>
<li>The columns are associated with the east-west direction.</li>
</ul>

Example: 

<div>
 <table>
 <tbody><tr>
 <td>E</td>
 <td>E</td>
 <td>S</td>
 <td>W</td>
 </tr>
 <tr>
 <td>N</td>
 <td>W</td>
 <td>S</td>
 <td>N</td>
 </tr>
 <tr>
 <td>S</td>
 <td>E</td>
 <td>E</td>
 <td>S</td>
 </tr>
 <tr>
 <td>E</td>
 <td>N</td>
 <td>W</td>
 <td>W</td>
 </tr>
 </tbody></table>


</div>

Expected Output: 6

Determine the minimum number of time steps required to get from the northwest corner to the southeast corner of a rectangular building.

Minimum Directional Path

You have been tasked with designing three classes: <code>text_editor</code>, <code>moving_text_editor</code>, and <code>smart_text_editor</code>. These classes are to be created with specific functionalities as defined below:

<ol>
<li><code>text_editor</code> class:

<ul>
<li><code>write_line(string:str)</code>: A method which appends a given string to the end of the existing string.</li>
<li><code>delete_line(char_num : int)</code>: A method which deletes <code>char_num</code> number of characters from the existing string, starting from the end. If there are no characters left, the method should do nothing.</li>
<li><code>special_operation()</code>: A method which currently does nothing.</li>
<li><code>get_notes()</code>: A method which returns the internal string.</li>
</ul></li>

<li><code>moving_text_editor</code> class:

<ul>
<li>This class extends <code>text_editor</code>. The <code>special_operation()</code> method is overridden. Initially, the cursor will be at the end of the current string. If <code>special_operation()</code> is called, it moves the cursor to the beginning of the string, any additional appends will be appended to the beginning of the string instead. Calling <code>special_operation()</code> again reverses the cursor operation.</li>
</ul></li>

<li><code>smart_text_editor</code> class:

<ul>
<li>This class extends <code>text_editor</code>. In this class, the <code>special_operation()</code> method is overridden to serve as an undo operation, allowing it to undo an infinite number of operations.</li>
</ul></li>
</ol>

<h3>Input</h3>

<pre tabindex="0" class="chroma"><code>[[&#39;special_text_editor&#39;], [&#39;write_line&#39;, &#39;special_operation&#39;, &#39;write_line&#39;], [&#39;World&#39;, &#39;Hello, &#39;]]
</code></pre>

<h3>Output</h3>

<code>&#34;Hello, World&#34;</code>

Create a text editor while displaying method overriding as a concept.

Text Editor With OOP

Design Patterns

Let’s say you’re tasked with building the YouTube video recommendation algorithm.

How would you design the recommendation system?

What are important factors to keep in mind when building the recommendation algorithm?

Let’s say that you’re in charge of getting payment data into your internal data warehouse.

How would you build an ETL pipeline to get Stripe payment data into the database so analysts can build revenue dashboards and run analytics?

Let’s say you’re a data engineer at a rapidly expanding e-commerce company, and the volume of daily transactions has increased from 10,000 to 100,000 over the past year.
How would you approach designing or adapting your data pipelines to ensure they remain both scalable and reliable as the business continues to grow?

Let’s say we’re launching a new smart fitness tracker that targets both casual users and dedicated athletes, and we want to understand the opportunity before going to market.

How would you estimate the potential market size, segment the customer base, identify main competitor types, highlight likely roadblocks, outline your high-level marketing approach, and walk through one specific marketing campaign you’d propose?

Let’s say we’ve proposed a new analytics dashboard to a client, but after reviewing our design and data sources, the client says the solution doesn’t meet their needs and pushes back on several key features.

How would you approach the conversation to negotiate alternative solutions, and what steps would you take to ensure disagreements are handled constructively so we can still reach a successful outcome?

Let’s say you work at Meta and currently Facebook allows users to specify whether or not a person is single or is in a relationship.

However, running through the reports shows that users with partners tend to make fewer posts. How would you approach tackling this issue? What might we want to do to increase engagement for this demographic?

How would you increase the user engagement of a certain demographic?

Romantic Reduction

Analytics

Let’s say your manager asks you to build a model with a neural network to solve a business problem.

How would you justify the complexity of building such a model and explain the predictions to non-technical stakeholders?

Which is the following domains is generally NOT a good use case for using a deep neural net?

Which is the following domains is generally NOT a good use case for using a deep neural net?

Neural Net Use Cases

What do you think are the most important metrics for WhatsApp?

What is the optimal strategy for reducing customer churn rate in a messaging platform like WhatsApp?

What is the optimal strategy for reducing customer churn rate in a messaging platform like WhatsApp?

Facebook’s revenue comes from ads displayed on the site. At the same time, Facebook strives to maintain the authenticity of content showcased to our users.

As a data scientist at Facebook, how would you decrease the prevalence of fake and problematic ads on the site, even if they brought substantial revenue to the company?

How would you decrease the prevalence of fake and problematic ads on the site?

Fake Ad Prevalence

Let’s say that you work at Reddit. The CEO wants to increase the overall engagement on Reddit. They decide that one way to do this is to build a “hot posts” or “trending posts” sort feature.

If you were in charge of leading the project, how would you approach the problem and build this feature?

You’re a data engineer and have been tasked with designing an app that allows users to find parking spots across a city. The app makes available parking spots across the city leased from various other organizations and companies.

You can assume that the list of available spots will be made available to you in a raw format, so you don’t need to worry about who owns which spot, for example.

How would you, as a data engineer, design the system that supports this application? You may make any needed assumptions, but be sure to identify them clearly.

Keep a few things in mind:

<ul>
<li>The app will have a machine learning component that updates prices in real time in response to demand. You don’t have to design the ML algorithm, but the app will have to use its outputs. Once a user commits to a spot, the per-hour price for that spot does not change.</li>
<li>The app needs to know the user’s location and should be able to return multiple nearby and available parking spots.</li>
<li>The app should be able to display to the user how much cost they’ve incurred so far once they’ve parked.</li>
<li>Be sure to identify the functional and non-functional requirements of the design.</li>
</ul>

Calculate Moving Average	SQL	Easy
Predict Customer Churn	Machine Learning	Medium
A/B Test Significance	Statistics	Medium
Optimize Query Performance	SQL	Hard
Feature Importance Analysis	Machine Learning	Medium
Clean Missing Data	Python	Easy
Neural Network Architecture	Deep Learning	Hard
Calculate Cohort Retention	SQL	Medium
Bayesian Probability	Statistics	Easy
Recommend Similar Products	Machine Learning	Hard

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