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Using coding interview challenges to hire software engineers


In an ever-changing market for tech talent, hiring the right software engineers can be a complex and demanding process. Identifying candidates with the right technical skill sets, assessing their problem-solving capabilities, and evaluating their fit for specific roles are all part of the challenge that companies face. Missteps in this process can lead to inefficient hiring, onboarding candidates who are not the right fit for the role or who lack the necessary skills, leading to wasted time, effort, and resources.

Leveraging coding interview challenges effectively can help address these challenges. These challenges offer a practical, hands-on way to assess a candidate’s technical skills, their approach to problem-solving, and their familiarity with key computer science and software development concepts. Additionally, they can help filter out candidates early in the process who lack the necessary technical skills, increasing the efficiency of your hiring process.

However, if not executed correctly, coding interview challenges can potentially create more problems than they solve. Poorly designed challenges can confuse candidates, fail to accurately assess necessary skills, or even discourage qualified individuals from continuing in the hiring process. Challenges that are too easy may not provide enough differentiation between candidates, while those that are too hard may exclude potentially successful hires who could excel with some on-the-job learning. Furthermore, without a standard measure for evaluating performance, coding challenges can introduce bias into the hiring process. 

To avoid these pitfalls, it’s crucial to engage a trusted vendor like CodeSignal in making use of predictive, fair, and scalable coding challenges. CodeSignal, a leading platform for conducting technical interviews and assessments, helps companies implement effective coding interview challenges in pre-screen assessments, live technical screenings, and on-site interviews. By leveraging their expertise and data-driven platform, you can ensure your coding challenges accurately assess candidate skills and help you make the right hires, faster.

What are coding interview challenges?

A coding interview challenge is a structured assessment designed to evaluate the technical skills of candidates for junior developer, senior-level software engineering, front-end developer, data analyst, machine learning engineer, and other technical roles. Typically, these challenges encompass problems that require knowledge of algorithms, data structures, and core software engineering principles. The candidate is asked to write code in an IDE and debug in a terminal to solve these problems.

Companies use these challenges as an integral part of the hiring process for engineers. They serve multiple purposes: verifying the candidate’s coding ability, assessing problem-solving skills, and understanding how an individual approaches complex tasks. The coding interview challenge not only evaluates whether the candidate can produce a correct and efficient solution but also explores their methods of planning, implementation, debugging, and optimization. These challenges contribute significantly to creating a comprehensive picture of a candidate’s capabilities, aiding companies in making informed hiring decisions.

Why do companies use coding challenges to interview developers?

Companies use coding challenges to interview developers because they offer an objective measure of a candidate’s technical knowledge and skills. These challenges require candidates to apply their knowledge in practical scenarios, including algorithm development, data structure utilization, and application of software engineering principles. Through coding challenges in assessments and interviews, recruiters can gain deeper insight into a candidate’s skills, providing a more accurate representation of their coding ability and their aptitude for problem-solving than resume screening can capture.

Moreover, coding challenges offer insights into a candidate’s approach towards complex tasks and problem-solving, beyond the simple measure of whether they can write functional code. Companies can gain an understanding of a candidate’s planning strategy, how they implement and debug their code, and their efficiency in optimizing solutions. This methodology offers a glimpse into how a candidate may perform in real-world work scenarios, making it a valuable tool in the decision-making process during hiring.

What are the different types of coding interview challenges?

In a technical interview, hiring teams are looking at a number of different areas to assess a candidate’s abilities. These can include their problem-solving skills using algorithms, knowledge of specific programming languages, proficiency in handling and analyzing data for data science tasks, understanding of object-oriented programming, and capacity to evaluate and refine existing code during code review exercises. Each of these tasks provides insight into a different aspect of a candidate’s coding skills and abilities.

Algorithmic challenges

Algorithmic challenges form a crucial component of the coding interview process. These challenges are designed to gauge a candidate’s problem-solving skills and their understanding of how to design and implement efficient algorithms. These types of challenges are important across all software engineering roles because they demonstrate the candidate’s ability to think logically, solve complex problems, and write efficient code. CodeSignal, with its validated pre-screen assessments and extensive library of coding challenges for live interviews, is an excellent tool for conducting these kinds of interviews. It allows hiring teams to assess a candidate’s ability to devise and implement algorithms effectively, providing valuable insight into their problem-solving abilities.

Programming language challenges

Programming language challenges are valuable when hiring for specialized and senior-level technical roles. These challenges are designed to assess a candidate’s fluency in a specific programming language, such as Python, Java, JavaScript, Ruby, or C++, and are particularly relevant for roles where expertise in a specific language is required. Understanding a candidate’s proficiency in these languages allows hiring teams to evaluate how effectively and efficiently they can write, debug, and maintain code in the context of the specific role. However, many organizations are most concerned with the correctness of a candidate’s solution, rather than the language, so hiring managers can also give candidates the option to choose. CodeSignal is a useful platform for conducting these challenges, as it allows hiring managers and interviewers to set up a variety of coding tasks in multiple languages, offering a comprehensive assessment of the candidate’s software development skills.

Data science challenges

Data science challenges are a specific category of tasks in the coding interview process that evaluate a candidate’s ability to handle, analyze, and draw insights from large data sets. These tasks are essential when interviewing for data science roles, where the ability to manipulate data and apply statistical analysis is key. They test a candidate’s skills in various areas, such as data cleaning, exploratory data analysis, data visualization, and the application of machine learning models. CodeSignal can assist in the process by offering a platform where these challenges can be created and conducted. The platform’s capacity to handle large datasets and compatibility with popular data science languages like Python and R, as well as an integrated Jupyter notebook, make it an ideal tool for conducting data science interviews.

Object-oriented programming challenges

Object-oriented programming (OOP) challenges are integral when hiring for roles that involve the development and maintenance of complex software systems. These challenges evaluate a candidate’s understanding of OOP principles such as encapsulation, inheritance, and polymorphism, and their ability to design software in a modular and scalable way. Assessing these skills gives hiring teams insight into a candidate’s ability to effectively structure and manage large codebases. CodeSignal is a valuable platform for conducting these OOP challenges, providing an environment where candidates can demonstrate their ability to design classes, create objects, and manage their interactions effectively.

Code review challenges

Code review challenges are useful when hiring for roles that involve maintaining and improving existing codebases. These tasks assess a candidate’s ability to read, understand, critique, and suggest improvements for existing pieces of code, which is crucial for ensuring code quality and maintainability. This kind of challenge tests not just a candidate’s coding skills, but also their understanding of best practices, coding standards, and their ability to work collaboratively on a codebase. CodeSignal offers a practical platform for conducting code review challenges, allowing hiring teams to evaluate how candidates interact with and propose improvements to real pieces of code, thus revealing their approach to collaborative coding and continuous code improvement.

How to design a coding interview challenge for your technical hiring needs

When designing a coding interview challenge to meet your company’s technical hiring needs, there are several key factors to consider. First and foremost, the challenge should be directly relevant to the technical skills you seek in a candidate. If the role is more data-oriented, challenges that require data manipulation and analysis skills would be suitable. If the position requires a deep understanding of algorithms, focus on problems that test the candidate’s algorithmic knowledge and problem-solving skills. 

Striking the right balance for difficulty level is also important; challenges should not be so easy that they fail to adequately test the candidate’s abilities, nor so difficult that they become demoralizing or unreasonably tough. Clear and concise instructions are crucial to prevent any confusion and to ensure the candidate understands the problem at hand. Finally, consider the time limit for the coding interview challenge—it should be sufficient for candidates to showcase their abilities without feeling unduly pressured. Here at CodeSignal, we’ve found that providing candidates 60 to 90 minutes to complete a technical assessment is the ideal timeframe to allow them to demonstrate their coding skills, without taking up too much of their time.

Beyond the content and difficulty of the coding challenge, designing an effective coding interview challenge involves broader considerations. Fairness and bias reduction should be at the forefront of the challenge design process; a well-designed challenge should not favor candidates with certain backgrounds over others. Also, consider the scalability of the challenge. As your company grows and the number of open technical roles increases, the coding challenge should be able to handle a larger volume of candidate submissions without losing its effectiveness. Additionally, ensure a good candidate experience by making the challenge engaging, relevant, and reasonable in its demands. 

With so many elements to consider in designing a coding interview challenge, a best-in-class tech hiring platform like CodeSignal can be a valuable asset. CodeSignal helps streamline the process of creating coding challenges that are fair, predictive, and scalable, while ensuring an optimal candidate experience. It takes into account the technical skills required for specific roles, the need to balance challenge difficulty, and the importance of clear instructions and appropriate time limits. Moreover, it can help design test cases that validate the correctness of solutions, ensuring an effective assessment of candidates’ skills.

How to use coding interview challenges to measure candidate skills

Evaluating a coding interview challenge goes beyond simply checking whether the candidate’s solution works or not. In addition to ensuring the correctness of the solution, it’s also important to assess the code’s efficiency: does the candidate’s solution make optimal use of resources such as memory and processing power? Code readability is another crucial factor: is the candidate’s code well-organized, logically structured, and properly documented? This is essential for maintainability and effective collaboration within a team. Additionally, the candidate’s problem-solving skills are key. How does the candidate approach the problem, break it down, and devise a solution? Evaluating these aspects provides a more comprehensive understanding of the candidate’s technical and cognitive abilities.

CodeSignal uses a validated, research-backed approach to evaluate these different aspects of a candidate’s skills. The platform uses patented, proprietary technology to assess the correctness and completeness of a candidate’s code, as well as deeper insights on code quality and skill area proficiency. This multifaceted evaluation process ensures a fair assessment of a candidate’s capabilities, helping to eliminate biases that may come from purely subjective evaluations.

Coding interview challenges for recruiting software engineers

Coding interview challenges can range from front-end assessments that test a candidate’s ability to create effective user interfaces, to programming language-specific challenges that gauge their proficiency in languages like JavaScript, Java, Python, Swift, React, and more. For those hiring junior developers, these challenges can be tailored to evaluate foundational knowledge and potential, helping you find the talent that will grow with your company.

Junior developer example

The following is a retired coding interview challenge from CodeSignal’s General Coding Framework, the industry standard for assessing junior developer skills at scale:

Given an array a, your task is to output an array b of the same length by applying the following transformation: 
– For each i from 0 to a.length - 1 inclusive, b[i] = a[i - 1] + a[i] + a[i + 1]
– If an element in the sum a[i - 1] + a[i] + a[i + 1] does not exist, use 0 in its place
– For instance, b[0] = 0 + a[0] + a[1]


For a = [4, 0, 1, -2, 3]
b[0] = 0 + a[0] + a[1] = 0 + 4 + 0 = 4
b[1] = a[0] + a[1] + a[2] = 4 + 0 + 1 = 5
b[2] = a[1] + a[2] + a[3] = 0 + 1 + (-2) = -1
b[3] = a[2] + a[3] + a[4] = 1 + (-2) + 3 = 2
b[4] = a[3] + a[4] + 0 = (-2) + 3 + 0 = 1

So, the output should be solution(a) = [4, 5, -1, 2, 1].

Taking a look at this question, you can see that it covers a basic array traversal and manipulation. The candidate simply needs to return the sum of each value in a, plus its right and left neighbors. 

At the same time, the question asks candidates to take into account corner cases with their implementation, which is an important fundamental skill. They need to correctly handle the first and last elements of the array. 

Pattern-matching example

The following is another retired coding interview challenge from CodeSignal’s General Coding Framework:

You are given two strings: pattern and source. The first string pattern contains only the symbols 0 and 1, and the second string source contains only lowercase English letters.

Your task is to calculate the number of substrings of source that match pattern

We’ll say that a substring source[l..r] matches pattern if the following three conditions are met:
– The pattern and substring are equal in length.
– Where there is a 0 in the pattern, there is a vowel in the substring. 
– Where there is a 1 in the pattern, there is a consonant in the substring. 

Vowels are ‘a‘, ‘e‘, ‘i‘, ‘o‘, ‘u‘, and ‘y‘. All other letters are consonants.


For pattern = "010" and source = "amazing", the output should be solution(pattern, source) = 2.
– “010” matches source[0..2] = "ama". The pattern specifies “vowel, consonant, vowel”. “ama” matches this pattern: 0 matches a, 1 matches m, and 0 matches a. 
– “010” doesn’t match source[1..3] = "maz" 
– “010” matches source[2..4] = "azi" 
– “010” doesn’t match source[3..5] = "zin" 
– “010” doesn’t match source[4..6] = "ing"

So, there are 2 matches. For a visual demonstration, see the example video

For pattern = "100" and source = "codesignal", the output should be solution(pattern, source) = 0.
– There are no double vowels in the string "codesignal", so it’s not possible for any of its substrings to match this pattern.

Guaranteed constraints:
1 ≤ source.length ≤ 103
1 ≤ pattern.length ≤ 103

This is a pattern-matching question where instances of a pattern need to be found inside of a larger array. It has the advantage of testing several fundamental programming skills at once: traversing multiple arrays with nested loops, working with subarrays, and performing basic collections/string operations.

Note that the guaranteed constraints in this question indicate that the candidate shouldn’t worry about optimizing their solution. 

Data structures example

Here is one more retired coding interview challenge from CodeSignal’s General Coding Framework, which evaluates candidates’ ability to work with and manipulate data structures, as well as deeper problem-solving abilities:

Given an array of unique integers numbers, your task is to find the number of pairs of indices (i, j) such that i ≤ j and the sum numbers[i] + numbers[j] is equal to some power of 2.
Note: The numbers 20  = 1, 21 = 2, 22 = 4, 23 = 8, etc. are considered to be powers of 2.


For numbers = [1, -1, 2, 3], the output should be solution(numbers) = 5.
– There is one pair of indices where the sum of the elements is 20 = 1: (1, 2): numbers[1] + numbers[2] = -1 + 2 = 1
– There are two pairs of indices where the sum of the elements is 21 = 2:(0, 0) and (1, 3)
– There are two pairs of indices where the sum of the elements is 22 = 4: (0, 3) and (2, 2)
– In total, there are 1 + 2 + 2 = 5 pairs summing to powers of 2.

For numbers = [2], the output should be solution(numbers) = 1.
– The only pair of indices is (0, 0) and the sum is equal to 22 = 4. So, the answer is 1.

For numbers = [-2, -1, 0, 1, 2], the output should be solution(numbers) = 5.
– There are two pairs of indices where the sum of the elements is 20 = 1: (2, 3) and (1, 4)
– There are two pairs of indices where the sum of the elements is 21 = 2: (2, 4) and (3, 3)
– There is one pair of indices where the sum of the elements is 22 = 4: (4, 4)
– In total, there are 2 + 2 + 1 = 5 pairs summing to powers of 2

Guaranteed constraints:
1 ≤ numbers.length ≤ 105
-106 ≤ numbers[i] ≤ 106

This problem could be solved in a straightforward way by having two nested loops to choose each pair and check whether their sum is a power of two. But since the numbers array could be quite large, quadratic time complexity would be too much for this question. (To get more precise, it is O(n2 * log(MAX_NUMBER)) where MAX_NUMBER is the largest number seen in the array.)

Therefore, this question tests whether candidates have the problem-solving and data structures skills to use a lookup table (hash set/dictionary) in their programming language of choice. It also involves a bit of tricky logic to avoid double-counting pairs. Finally, this question asks candidates to pay close attention to constraints, testing a key skill for real-world development.

Scaling coding interview challenges with CodeSignal

For streamlining your technical hiring process, CodeSignal is an indispensable tool. Whether you’re hiring early-career developers, senior-level engineers, or data scientists for technical roles, CodeSignal’s platform helps you conduct, scale, and measure the results of your coding interview challenges efficiently. CodeSignal’s Certified Evaluations are role-relevant assessments that are written and maintained by subject matter experts and validated by our in-house Industrial-Organizational (IO) Psychologists. Certified Evaluations include dynamic rotation of coding challenges, reducing the risk of cheating and providing a stronger, more reliable signal of candidates’ skills. 

Measuring the results of coding interview challenges

In addition to its flexibility and scalability, CodeSignal also provides a comprehensive Coding Report to help you dig deeper into a candidate’s proficiency in coding interview challenges. By assessing not only the correctness of the candidates’ solutions but also their proficiency in specific skill areas, it offers a more detailed breakdown of a candidate’s qualifications for a role. This thorough analysis helps you make informed hiring decisions based on objective data, ensuring more effective live coding interviews, as well as a fair and efficient recruitment process.

Final thoughts

In conclusion, leveraging coding interview challenges effectively is key to hiring the right software engineers. However, it’s crucial to engage a trusted vendor like CodeSignal to ensure these challenges are designed and implemented in a way that helps your organization make the right hires, faster—and with fewer resources. With the right approach and tools, coding interview challenges can streamline your hiring process, ensuring you find and hire the most qualified technical talent.

Ready to see the CodeSignal platform in action? Schedule a call with one of our technical hiring experts today.

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