# Software Engineer
**Role:** Software Engineer · **Level:** L5
**Company:** [Citadel](https://scaleengineer.com/companies/citadel)
**Difficulty:** Medium to Hard
**Salary:** US$140000 - US$180000
**Experience:** 5 - 8
**Timeline:** ~14 days
This interview process is for a Software Engineer position at Citadel, specifically for the L5 level. It is designed to assess a candidate's technical skills, problem-solving abilities, and cultural fit within the company.
Canonical: https://scaleengineer.com/interviews/citadel/l5-software-engineer
---
## Overall evaluation

- Technical Skills
- Behavioral and Soft Skills
- Software Development Practices

## Questions asked

- Given an array of integers, find the contiguous subarray with the largest sum.
- Design a system to handle real-time stock price updates for millions of users.
- Explain the concept of ACID properties in database transactions.
- Tell me about a time you faced a significant technical challenge and how you overcame it.
- How would you optimize a slow database query?
- Describe the difference between concurrency and parallelism.
- What are the trade-offs between using a relational database and a NoSQL database?
- How do you ensure the quality of the code you write?
- Describe a situation where you had to work with a difficult team member.

## Preparation tips

### lists

- Review fundamental computer science concepts: data structures, algorithms, operating systems, databases.
- Practice coding problems on platforms like LeetCode, HackerRank, or AlgoExpert.
- Study system design principles and common architectural patterns.
- Prepare behavioral questions using the STAR method.
- Research Citadel's business, culture, and recent news.
- Understand the specific technologies and languages mentioned in the job description.

### studyPlan

- {"title":"Data Structures and Algorithms","longDescription":"Weeks 1-2: Focus on core data structures (arrays, linked lists, trees, graphs, hash maps) and algorithms (sorting, searching, dynamic programming, graph traversal). Practice implementing these from scratch and analyze their time/space complexity. Aim for at least 5-10 problems per day.","shortDescription":"Weeks 1-2: Data Structures & Algorithms fundamentals. Practice 5-10 problems daily."}
- {"title":"System Design","longDescription":"Weeks 3-4: Dive into system design. Study concepts like scalability, availability, consistency, load balancing, caching, databases (SQL vs. NoSQL), message queues, and microservices. Work through common system design case studies.","shortDescription":"Weeks 3-4: System Design principles and case studies."}
- {"title":"Behavioral and OOP","longDescription":"Week 5: Prepare for behavioral questions. Identify key experiences that demonstrate leadership, teamwork, problem-solving, and conflict resolution. Practice articulating these using the STAR method. Also, review object-oriented programming (OOP) principles.","shortDescription":"Week 5: Behavioral questions (STAR method) and OOP."}
- {"title":"Mock Interviews and Review","longDescription":"Week 6: Mock interviews. Simulate the interview environment with peers or online platforms. Focus on communication, clarity of thought, and time management. Review any weak areas identified during mock interviews.","shortDescription":"Week 6: Mock interviews and final review."}

## Location differences

- {"location":"New York","differences":{"tips":["Brush up on distributed systems concepts like consensus algorithms, replication, and partitioning.","Be prepared to discuss your experience with cloud platforms (AWS, Azure, GCP).","Practice system design problems related to high-throughput and low-latency applications."],"interviewFocus":["Deep understanding of distributed systems and their trade-offs.","Experience with large-scale data processing and low-latency systems.","Ability to design and analyze complex algorithms and data structures."],"commonQuestions":["Discuss a challenging technical problem you solved in a distributed system.","How would you design a real-time bidding system for online advertising?","Explain the CAP theorem and its implications for distributed databases."]}}
- {"location":"Chicago","differences":{"tips":["Focus on practicing LeetCode-style problems, especially those involving arrays, strings, trees, and graphs.","Understand time and space complexity analysis (Big O notation).","Be ready to explain your thought process clearly and concisely."],"interviewFocus":["Strong foundation in data structures and algorithms.","Proficiency in at least one major programming language (Java, Python, C++).","Problem-solving and analytical skills."],"commonQuestions":["Describe a time you had to optimize a piece of code for performance.","How would you design a URL shortening service?","Explain the difference between a process and a thread."]}}
- {"location":"London","differences":{"tips":["Prepare specific examples using the STAR method (Situation, Task, Action, Result).","Research Citadel's mission, values, and recent projects.","Be enthusiastic and demonstrate genuine interest in the role and company."],"interviewFocus":["Teamwork and collaboration skills.","Communication and interpersonal abilities.","Alignment with Citadel's values and culture."],"commonQuestions":["Tell me about a project you are particularly proud of and your role in it.","How do you handle disagreements within a team?","What are your thoughts on agile development methodologies?"]}}

## Round 1: Technical Coding Round 1
**Type:** Data Structures and Algorithms Interview · **Difficulty:** Hard · **Duration:** 45 min
Coding challenge focusing on data structures and algorithms.
This round focuses on your core data structures and algorithms knowledge. You will be presented with one or two coding problems and expected to write clean, efficient code on a whiteboard or shared editor. The interviewer will assess your ability to analyze the problem, devise a solution, discuss trade-offs, and implement it correctly, including handling edge cases.
**Interviewers look for:** Strong problem-solving skills.; Proficiency in coding.; Logical thinking.; Ability to break down complex problems.
**Evaluation criteria:** Correctness of the solution.; Efficiency of the solution (time and space complexity).; Clarity and organization of the code.; Ability to handle edge cases.; Communication of the thought process.
**Common rejection reasons:** Inability to articulate thought process clearly.; Incorrect or inefficient algorithmic solutions.; Poor understanding of time and space complexity.; Lack of attention to edge cases.
## Questions

- Find the kth largest element in an unsorted array.
- Implement a function to check if a binary tree is a valid Binary Search Tree.
- Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

## Preparation tips

- Practice coding problems extensively.
- Understand Big O notation for time and space complexity.
- Be prepared to explain your approach before coding.
- Test your code thoroughly with various inputs, including edge cases.

## Round 2: System Design Round
**Type:** System Design Interview · **Difficulty:** Hard · **Duration:** 60 min
Design a scalable system based on a given problem.
This round assesses your ability to design complex, scalable systems. You'll be given an open-ended problem (e.g., design Twitter's feed, design a URL shortener) and expected to discuss various components, data models, APIs, and trade-offs. The focus is on your architectural thinking and ability to handle scale and reliability.
**Interviewers look for:** System design thinking.; Knowledge of distributed systems.; Ability to make reasoned technical decisions.; Experience with large-scale systems.
**Evaluation criteria:** Scalability of the proposed design.; Availability and reliability considerations.; Choice of appropriate technologies and data stores.; Understanding of trade-offs (e.g., consistency vs. availability).; Ability to handle failures and edge cases.
**Common rejection reasons:** Inability to design a scalable and robust system.; Lack of understanding of distributed systems concepts.; Poor consideration of trade-offs.; Failure to address potential failure points.
## Questions

- Design a rate limiter.
- Design a distributed cache.
- Design a system to count unique visitors to a website.

## Preparation tips

- Study common system design patterns.
- Understand concepts like load balancing, caching, databases, and message queues.
- Practice designing various popular applications.
- Be prepared to justify your design choices.

## Round 3: Behavioral and Managerial Round
**Type:** Behavioral Interview · **Difficulty:** Medium · **Duration:** 45 min
Assesses behavioral competencies and cultural fit.
This round focuses on your behavioral and situational responses. You'll be asked questions about your past experiences, how you handle challenges, work in teams, and your career aspirations. The goal is to understand your personality, work ethic, and how you would fit into the team and company culture.
**Interviewers look for:** Cultural fit.; Teamwork capabilities.; Communication skills.; Motivation and passion.
**Evaluation criteria:** Communication skills.; Teamwork and collaboration.; Problem-solving approach.; Adaptability and learning attitude.; Alignment with company values.
**Common rejection reasons:** Lack of clear communication.; Inability to provide specific examples.; Poor cultural fit.; Lack of enthusiasm or interest.
## Questions

- Tell me about a time you failed and what you learned from it.
- Describe a situation where you had to work with a difficult colleague. How did you handle it?
- Why are you interested in working at Citadel?

## Preparation tips

- Prepare specific examples using the STAR method.
- Research Citadel's values and culture.
- Be honest and authentic in your responses.
- Ask thoughtful questions about the team and role.