Must-Know Algorithms for Coding Interviews in 2024: Insider Tips
Tags: interview, beginners, webdev, career
Published: 2024-02-02 15:15:00

Let's discuss some of the most important [algorithms](https://devinterview.io/questions/data-structures-and-algorithms/) to be proficient in for the coding interview. Having done more than a hundred interviews for big tech companies and going through many interviews myself, I thought I would share some of my best advice with you as you are preparing for your next interview.

There are a lot of different algorithms to study and to know, but I thought I would I would highlight a few for you to pay particular attention to, especially one of the top algorithms to know is tree traversal. Tree traversal is used extensively in many projects, and I would be very surprised if you went through an interview without being asked at least one tree traversal question. There aren't really too many difficult algorithms but one that you should be really well versed in is tree traversal, among the others that you must know as well.

## Tree Traversal

Tree Traversal not only shows to one's ability to navigate through a [tree data structure](https://devinterview.io/questions/data-structures-and-algorithms/tree-data-structure-interview-questions/) but also is an indicator of understanding complex relationships and operations within hierarchical systems. Its relevance extends to a wide range of applications, including more complex algorithmic problems and file system and [database operations](https://devinterview.io/questions/software-architecture-and-system-design/databases-interview-questions/).

### Types of Tree Traversal

When discussing tree traversal, it's crucial to familiarize yourself with the primary methods of traversal:

### Preorder Traversal

In this method, you visit the root node first, then recursively do a preorder traversal of the left subtree, followed by a recursive preorder traversal of the right subtree. Ideal for creating a copy of the tree. By visiting nodes before their descendants, it preserves the hierarchy in a straightforward manner, facilitating cloning processes or tree-based expressions serialization.

### Inorder Traversal

This involves first doing a recursive inorder traversal of the left subtree, then visiting the root node, and finally, doing a recursive inorder traversal of the right subtree. Particularly useful in [binary search trees](https://devinterview.io/questions/data-structures-and-algorithms/binary-tree-data-structure-interview-questions/) (BST), where inorder traversal yields nodes in non-decreasing order. This property becomes crucial in tasks like sorting and searching in BSTs.

### Postorder Traversal

Here, you first recursively traverse the left and right subtrees, and then visit the root node. Finds its use in the deletion of the tree. Since it visits the parent node after the children, it ensures that subtrees are properly deleted or freed before the parent, maintaining integrity and preventing memory leaks.

### Breadth-first Search (BFS)

This method involves traversing the tree level by level from top to bottom. Beyond level-order traversal, BFS is helpful in shortest path problems where the tree models states or configurations. It’s optimal for finding the shortest sequence of actions from the root to a target node.

### Depth-first Search (DFS)

Unlike BFS, depth-first search involves exploring as far down a branch as possible before backtracking and exploring other branches. It can be useful in solving puzzles and games formulated as trees, where finding a viable or optimal solution requires exploring an exhaustive or pruned search space.

### Practical Application

A common interview problem that showcases the utility of tree traversal is the **view hierarchy problem**, where you're given a hierarchy of views with subviews and tasked with traversing every view to print it out. This scenario can be effectively modeled as a tree problem, illustrating the need for both recursive and iterative approaches to tree traversal.

The versatility of tree traversal techniques extends to advanced challenges like **balanced tree validations**, where utilizing inorder traversal can help verify the BST property. Similarly, problems requiring the enumeration of paths or subtrees fitting specific criteria can leverage DFS for efficient exploration.

Implementing tree traversal also hints at an engineer's capacity to optimize and adapt. Iterative implementations using [stacks](https://devinterview.io/questions/data-structures-and-algorithms/stack-data-structure-interview-questions/) (for DFS) or [queues](https://devinterview.io/questions/data-structures-and-algorithms/queue-data-structure-interview-questions/) (for BFS) showcase an understanding of alternative approaches beyond recursion, which can be critical in environments with stack size limitations.

Moreover, integrating tree traversal algorithms with other data structures, like [graphs](https://devinterview.io/questions/data-structures-and-algorithms/graph-data-structure-interview-questions/) represented through adjacency lists or matrices, underlines the adaptability of these methods in solving complex, interconnected data problems.

## Recursion

[Recursion](https://devinterview.io/questions/data-structures-and-algorithms/recursion-algorithm-interview-questions/) is a fundamental concept in computer science, widely utilized for its elegant approach to solving problems that can be divided into smaller, similar problems. It's particularly prevalent in tree traversals and various algorithms because it mirrors the natural, hierarchical structure of trees—each subtree can be considered a smaller instance of the tree itself.

Recursion hinges on a function calling itself with adjusted arguments, aiming to simplify a complex problem into tractable sub-problems. The technique is closely associated not just with tree traversal but also with solving puzzles like the Tower of Hanoi, navigating mazes, and processing nested structures common in computer science.

Yet, to use recursion effectively, you need a nuanced understanding of it. Recursive solutions can often grow unmanageable because they often amass large numbers of parameters to keep state between recursive calls, leading to code that is difficult to read and maintain.

Integrating helper functions is a strategic approach to streamline recursive solutions. These functions can neatly encapsulate the initialization of state and other preparatory steps, leaving the recursive function to focus on the core logic. Understanding the base case is equally important. The base case acts as an anchor, providing a clear exit point for the recursive calls to prevent the infinite recursion and potentially crash the program.

Remember that recursion is not a silver bullet for all [algorithmic problems](https://devinterview.io/questions/data-structures-and-algorithms/). Recursive calls consume stack space—each call adds a new layer to the call stack. Therefore, recursion, when applied to significantly deep recursions or extensive datasets, can lead to `StackOverflowError` or similar issues, as the system's stack space gets exhausted.

### Transition from Recursive to Iterative Solutions

In practice, professional developers often refactor recursive logic into iterative solutions to enhance performance and avoid stack overflow concerns. Iterative algorithms, which use loops to repeat operations, provide granular control over memory consumption and are good at processing larger datasets without the risk of stack overflow. Making this shift typically involves employing data structures like stacks or queues to explicitly manage what the recursive call stack would implicitly manage.

Experimenting with converting recursive algorithms to their iterative counterparts can significantly enhance your understanding of both approaches. For example, iterating a [Fibonacci sequence](https://devinterview.io/questions/data-structures-and-algorithms/fibonacci-sequence-interview-questions/) or tree traversal iteratively can reveal insights into state management and algorithm optimization. Such exercises also prepare you for interviews where the iterative solution might be preferred for its efficiency, despite the recursive solution's elegance and simplicity.

## Stacks and Queues in Interview Problem Solving

Stacks and queues frequently emerge in coding interviews as fundamental data structures that underpin many efficient algorithms. Their utility in solving a variety of problems, from simplifying complex operations to streamlining data processing tasks.

[Stacks](https://devinterview.io/questions/data-structures-and-algorithms/stack-data-structure-interview-questions/) operate on a Last In, First Out (LIFO) principle, making them well suited for scenarios where the most recent additions need to be accessed first. This unique characteristic lends itself to a variety of applications.

### Balancing Parentheses

One classic use-case for stacks is in verifying the correctness of expressions containing various types of parentheses or brackets. By pushing opening brackets onto the stack and popping them off when a corresponding closing bracket is encountered, one can efficiently track whether parentheses are balanced throughout the expression. This method ensures that each opening bracket is properly paired and closed in the correct order, mirroring the nested structure of well-formed expressions.

### Function Calls and Recursion

Stacks also play a role in managing function calls within most programming languages, where the call stack maintains an ongoing record of nested function calls. This functionality underscores the importance of understanding stack dynamics when designing recursive functions or simulating nested processes in your algorithms.

## Queues in Data Processing

[Queues](https://devinterview.io/questions/data-structures-and-algorithms/queue-data-structure-interview-questions/), with their FIFO (First In, First Out) behavior, are essential for algorithms that require processing elements in the order they were added. Their application ranges from breadth-first search in tree traversal to managing tasks in a scheduler, showcasing their versatility across different domains.

Most modern programming languages offer built-in support for stacks and queues, typically through their standard library collections. Familiarity with these implementations, including methods for adding, removing, and inspecting elements, is crucial for effectively leveraging these data structures in solutions.

When faced with a new problem, considering whether stacks or queues can simplify the solution is a worthwhile strategy. Their ability to manage ordered data efficiently often makes them ideal candidates for optimizing algorithms, particularly those involving sequential processing or hierarchical data.

## Object-Oriented Programming in Interviews

[Object-oriented programming](https://devinterview.io/questions/web-and-mobile-development/oop-interview-questions/) (OOP) is a paradigm frequently explored in coding interviews due to its critical role in software development. Understanding OOP concepts is not just about knowing how to write classes or methods; it's about demonstrating the ability to organize code into clean, reusable components that model real-world entities or abstract concepts effectively.

**Classes** serve as the blueprint for creating objects, encapsulating data, and behavior. In interviews, you may be asked to design a class from scratch to solve a problem or implement specific functionality. This tests your understanding of key OOP principles such as encapsulation, abstraction, inheritance, and polymorphism.

Your will need to demonstrate your ability to define and manipulate class members, including methods and variables. Knowing when to use private versus public visibility, static methods, or instance methods can significantly impact the design and functionality of your code. These decisions play an important role in data encapsulation and the interface your class exposes to the outside world.

### Application in Problem Solving

An interviewer might evaluate how you approach problem-solving using OOP by asking you to model a real-world system or a complex data structure. For instance, instead of using a basic two-dimensional [array](https://devinterview.io/questions/data-structures-and-algorithms/array-data-structure-interview-questions/) to represent a grid, you could define a `Grid` class. This class could encapsulate grid-related behaviors, such as adding or retrieving elements, thereby making your solution more modular, readable, and maintainable.

## Mastering HashMap Interview Problems

[HashMaps](https://devinterview.io/questions/data-structures-and-algorithms/heap-and-map-data-structures-interview-questions/) are a fundamental data structure that you must be proficient with to excel in coding interviews. They offer efficient data retrieval and are often the backbone of solutions requiring quick lookups.

At their core, hashmaps map keys to values, allowing for fast data retrieval based on keys. This is achieved through a hashing function that converts a key into an index where the value is stored. [Understanding the time-space trade-offs](https://devinterview.io/questions/data-structures-and-algorithms/big-o-notation-interview-questions/) involved in using hashmaps is essential, as they can significantly affect the efficiency of your solution.

### Common Uses in Interviews

Many algorithms can be optimized using hashmaps. For instance, a frequent interview question involves finding two numbers in an array that sum up to a specific target. Hashmaps can solve this problem in linear time by storing numbers and their indices as key-value pairs, thereby allowing for quick lookups to check if the complement of the current number (target - current number) exists in the array.

Going beyond simple key-value pair storage, you might need to create more complex data structures using hashmaps, such as hashmaps of hashmaps, or design custom hashing functions for unique problem scenarios. Demonstrating your ability to manipulate and extend hashmaps in these ways can significantly impress your interviewers.

If you find yourself stuck on a problem during an interview, consider whether a hashmap can provide an efficient solution. Its versatility in solving a wide range of problems makes it a powerful tool in your problem-solving arsenal. Practice implementing hashmaps in various scenarios to deepen your understanding and agility in using them under different problem constraints.

## Dynamic Programming in Coding Interviews

[Dynamic programming](https://devinterview.io/questions/data-structures-and-algorithms/dynamic-programming-interview-questions/) is a key technique for complex problem-solving in coding interviews. It simplifies problems by dividing them into manageable subproblems, solving each once, and storing their solutions, commonly in a hash map or array, for quick access later.

This approach is useful for problems that involve repetitive calculation, like computing [Fibonacci numbers](https://devinterview.io/questions/data-structures-and-algorithms/fibonacci-sequence-interview-questions/) or finding the shortest path in a graph. By storing intermediate results, dynamic programming saves time on calculations that would otherwise have exponential complexity.

### Memorization Technique

Memoization is a fundamental technique in dynamic programming. It saves results from expensive function calls and reuses them when the same inputs occur again, making recursive algorithms much more efficient by preventing repeated calculations.

Dynamic programming might seem intimidating because it requires figuring out the optimization strategy. However, it's valuable for solving problems where direct approaches don't work well. Demonstrating dynamic programming skills can make a difference in interviews, especially for optimization problems or when you need to navigate through data efficiently.

### Tips for Mastering Dynamic Programming

- **Break Down the Problem:** Start by understanding the main problem and identifying its subproblems. Recognizing these can help tackle the larger issue step by step.
- **Choose a Memoization Structure:** Deciding on the right way to store results, like using an array or hash map, is vital for accessing previous calculations efficiently.
- **Create State Transition Equations:** Define how you can build the solution to the main problem from the solutions to its subproblems.
- **Practice Regularly:** Getting better at dynamic programming requires practice. Begin with basic problems and gradually take on more complex ones to improve your skill.

## The Strategic Approach to Problem Solving in Interviews

Solving coding problems in interviews is not just about writing code; it's about demonstrating a thoughtful, analytical approach to problem-solving. This involves understanding the problem deeply, exploring various solutions, assessing their trade-offs, and choosing the most appropriate one based on the constraints.

### Analyzing the Problem

Begin by thoroughly understanding the problem statement. If anything is unclear, ask questions. Knowing the details can lead you to think of different solutions and understand their complexities.

Don't jump to the first solution that comes to mind. Instead, consider different algorithms or data structures that could solve the problem. Discuss the pros and cons of each alternative, considering factors such as time complexity, space complexity, and the readability of your code.

Many problems have several different solutions, each with its trade-offs between execution time and memory usage. Highlighting these trade-offs and justifying your choice of solution based on the problem's constraints demonstrates a sophisticated understanding of [software engineering principles](https://devinterview.io/questions/software-architecture-and-system-design/).

## Communicating Your Thought Process

Ultimately, the goal is not just to solve the problem but to convey your analytical and decision-making process clearly. Articulate why you chose a particular approach, how you optimized it, and what trade-offs you considered. This communication skill is often as important as the technical solution itself in interviews.

Thanks for reading. Best of luck in your technical interview!

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