Ace Your Coding Interview: A Comprehensive Guide for College Recruitment with Top Questions and Solutions
Jobs in software development are growing faster than average, and tech companies are specifically looking for people who are skilled at writing code. Coding questions are an integral part of an interview for a developer position. A Stack Overflow study reports that developers with strong coding skills often get more job opportunities and earn better salaries. Preparing for coding interviews requires a deep understanding of key concepts and problem-solving skills. This article will discuss coding interview questions you should know to crack those interviews and get your dream job.
Mastering Data Structures
Mastering data structures is essential for writing well-optimized and scalable code to solve complex problems. Understanding how data is stored, accessed, and altered helps developers build faster and more efficient solutions across diverse applications.
- Data Structures: A data structure is a storage format that defines how data is stored, organized, and manipulated. Data structure makes it easy for data to be accessed and used efficiently.
- Arrays: An array is a data structure that stores a fixed-size sequence of elements. All the items stored belong to the same type (list of numbers or names).
- Graphs: A graph in a data structure contains a set of ordered pairs, also known as edges or arcs.
- Trees: A tree is a hierarchical data structure used to represent data in a way that allows for easy traversal and organization.
- Linked Lists: A linked list is a linear data structure in which the elements are not necessarily stored in a contiguous manner. It is a sequence of nodes containing a value and a reference to the next node. Doubly linked lists are a particular type of linked list in which traversal across the data elements can be done in both directions.
- Stacks: A stack in a data structure performs operations in a LIFO (Last In First Out) order. Stack follows a Last In First Out (LIFO) pattern.
- Queues: A queue in a data structure performs operations in a FIFO (First In First Out) order. A deque is a double-ended queue.
- Binary Trees: A binary tree is an extension of the linked list structure where each node has at most two children.
Object-Oriented Programming (OOP) Concepts
OOPS stands for Object-Oriented Programming System, a paradigm that provides concepts such as objects, classes, and inheritance. It helps organize and structure code to represent real-world entities, their attributes, and the actions they can perform.
- Object: A real-world entity having a particular state and behavior.
- Inheritance: A concept that refers to an object gaining all the properties and behaviors of a parent object.
- Polymorphism: A concept that allows a task to be performed differently.
- Abstraction: A concept that hides the internal details of an application and only shows the functionality.
Essential Algorithms
Algorithms provide a structured approach to finding refined solutions, solving complex problems, and proving logical thinking. They assist developers with efficient problem-solving abilities in programming interviews.
There are many types of sorting algorithms: bubble sort, quick sort, balloon sort, merge sort, radix sort, and more. On average, the time complexity to implement quicksort is O(n log n).
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Recursion Explained
Recursion refers to a function calling itself based on a terminating condition. Multiply the number by the factorial of the number just before it until it reaches the base case (1 or 0).
Common Coding Problems and Solutions
We have organized the questions and answers into specific categories to guide your preparation.
Array Manipulation
- Reverse an Array in Place: To reverse an array in place, you can use two pointers: one starting from the beginning and the other from the end of the array.
- Find the K-th Largest Element: One way to find the K-th largest element is by sorting the array and selecting the element at the K-th position from the end.
- Move Non-Zero Elements to the Left: You can solve this by iterating over the array and shifting non-zero elements to the left.
Linked List Challenges
- Detect a Cycle in a Linked List: Using Floyd’s Cycle-Finding Algorithm, the tortoise and hare algorithm, you can detect a cycle in a linked list. You use two pointers: one moves one step at a time (slow), and the other moves two steps at a time (fast).
Stack and Queue Implementations
- Implement a Stack Using Two Queues: You can implement a stack using two queues: one for pushing elements and the other for popping. When pushing, enqueue the new element into the first queue.
- Evaluate a Postfix Expression: To evaluate a postfix expression, you use a stack.
- Implement a Stack with Min Retrieval: You use an auxiliary stack that stores the minimum values to implement a stack that supports retrieving the minimum element in constant time.
- Implement a Queue Using Two Stacks: To implement a queue using two stacks, you use one stack for enqueueing (pushing elements) and the other for dequeueing (popping elements).
- Circular Queue: A circular queue (or a ring buffer) is an extended version of a linear queue. It connects the last position back to the first to form a circle. It follows the First-in-First-out principle and helps utilize space.
Tree Traversal and Search
If both values are smaller/larger than the root, move left/right. BFS analyzes nodes level by level, making finding the shortest path in an unweighted graph simple.
Graph Traversal
- Undirected Graph (using DFS): You start DFS from any node and keep track of visited nodes and their parent.
- Directed Graph (using DFS and recursion stack): You have to use two sets: one for visited nodes and one for the recursion stack.
The default PriorityQueue is implemented as a min-heap.
Rotated Sorted Array
A rotated sorted array is like a sorted array that has been shifted.
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Knapsack Problem
In the 0/1 knapsack problem, you decide whether to include each item in a bag with limited capacity to maximize total value.
Coin Change Problem
Dynamic programming solves the coin change problem by finding the minimum number of coins using a 1D array. return dp[amount] > amount ?
N-Queens Problem
We can use backtracking to solve the N-Queens problem. Backtracking tries a number, moves forward, and backtracks if the placement is invalid.
Power Set
To find all subsets (power set), backtrack through each element.
System Design Fundamentals
System design interviews assess your ability to create scalable and reliable systems.
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- URL Shortening Service: We need a system that generates unique short URLs for long URLs to design a URL shortening service.
- Distributed File System: A distributed file system stores files across multiple machines to ensure redundancy, scalability, and high availability.
- Library Management System: A library management system stores information about books, members, and transactions.
- Movie Ticket Booking System: A movie ticket booking system stores movie details, show timings, bookings, and payment information.
- E-commerce Platform Backend: The backend for an e-commerce platform consists of entities like Users, Products, Orders, and Payments.
- Routing Requests: A round-robin or least-connections method can be used for routing requests.
- Real-Time Updates: The data can then be broadcast to clients through WebSockets or similar technologies to push the updates instantly.
- High-Frequency Trading System: Low latency is critical for high-frequency trading, so focus on designing a system that minimizes network hops and ensures fast execution. Use highly optimized, low-latency programming languages like C++ or Rust, and implement in-memory data stores for quick access to market data.
- Fault-Tolerant Database System: To design a fault-tolerant database system, replication is used to ensure data availability in case of a failure. Master-slave or multi-master replication can be set up to distribute data across multiple nodes. A backup and recovery plan should be in place to restore data from a recent snapshot in case of catastrophic failure.
Top Coding Interview Questions by Category
Here are the top coding interview questions that you must practice to increase your chances of success in interviews. These questions are categorized by different topics for better preparation. For a quick and focused revision in DSA, check out our GfG-160Master Complete DSA from basic to advanced with DSA-360°
Mathematical Problems
- Print the pattern (You only need to write the function here)
- Print table
- Series AP
- Series GP
- Closest Number
- Armstrong Numbers
- Sum of digits of a number
- Reverse digits
- Print the Kth Digit
- Binary number to decimal number
- Jumping Numbers
- GCD of two numbers
- LCM of two numbers
- Add two fractions
- GCD of array
- Factorial of a number
- Compute nPr
- Compute nCr
- Largest prime factor
- Perfect Numbers
- Pair cube count
- Find Nth root of M
- Prime Number
- Sieve of Eratosthenes
- Sum of all prime numbers between 1 and N
- Pairs of prime numbers
Related Learning Resources: Mathematical Algorithms Number Theory Puzzles Problems
Puzzles Problems
- Count Squares
- 3 Divisors
- Check if four points form a square
- Check for power
- Overlapping rectangles
- Trailing zeroes in factorial
- Angle between hour and minute hand
- Number Of Open Doors
- Triangular Numbers
- Nth Even Fibonacci Number
- Last two digit Fibonacci
- Squares in a Matrix
- Day of the week
Related Learning Resources: Puzzles
Arrays Problems
- Array operations (Search, insert, delete)
- Array alternate printing
- Maximum and minimum in an array
- Second largest in array
- Sum of array elements
- Reverse an Array
- Rotate Array
- Count of smaller elements
- Remove duplicate elements from sorted Array
- Count possible triangles
- Leaders in an array
- Minimum distance between two numbers
- Sorted subsequence of size 3
- Maximum Sub Array
- Majority Element
- Wave Array
- Maximum Index
- Max sum path in two arrays
- Product array puzzle
- Find duplicates in a small ranged array
- Find Missing And Repeating
- Stock buy and sell
- Trapping Rain Water
- Pair with given sum in a sorted array
- Chocolate Distribution Problem
- Longest Consecutive subsequence
- Three way partitioning
Related Learning Resources: Array Data Structure
String Problems
- Check for palindrome
- Check for anagram
- Anagram
- Palindrome
- Title case conversion
- Sort the string
- Merge two strings
- Save Ironman
- Good or Bad string
- Extract Maximum
- Reverse words in a given string
- Implement strstr
- Check for subsequence
- Check for rotation
- Check if two strings are k-anagrams
- Uncommon characters
- Anagram Search
- First repeating character
- First non-repeating character
- Longest Distinct characters in string
- Longest Palindromic Substring
- Find k-th character in string
- Smallest window with all characters
- Add Binary Strings
- Multiply two Strings
- Nearest multiple of 10
Related Learning Resources: String Data Structure
Searching Problems
- Linear Search
- Facing the sun
- Magnet Array Problem
- Binary Search
- Floor in a Sorted Array
- Count occurrences in a sorted array
- Search in a sorted and rotated
- Find the missing number
- Missing element of AP
- Square root of a number
- Transition Point in a Sorted Binary Array
- Last index of One
- Peak element
- Allocate minimum number of pages
- Common elements in three sorted
- Smallest Positive missing number
Related Learning Resources: Searching Algorithms
Sorting Problems
- Check if array is sorted
- Sort a binary array
- Sort an array of 0s, 1s and 2s
- Bubble Sort
- Insertion Sort
- Selection Sort
- Quick Sort
- Merge Sort
- Sort an array when two halves are sorted
- Relative Sorting
- Triplet Sum in Array
- Minimum Swaps to Sort
- Sorting elements by frequency
- Triplet Family
- Count the triplets
Related Learning Resources: Sorting Algorithms
Hashing Problems
- Count distinct elements
- Array Subset of another array
- Nuts and Bolts Problem
- Count frequencies of elements
- Check if two arrays are equal or not
- First element to occur k times
- In First But Second Non-Repeating Element
- Group Anagrams Together
- Winner of an election
- Check for a pair with given sum
- Count distinct pairs with difference k
- Count pairs with given sum
- Find all four sum numbers
- A Simple Fraction
- Largest Fibonacci Subsequence
Related Learning Resources: Hashing Data Structure
Matrix Problems
- Transpose of Matrix
- Print Matrix in snake Pattern
- Print a given matrix in spiral form
- Is Sudoku Valid
- Count zeros in a sorted matrix
- Squares in a Matrix
- A Boolean Matrix Question
- Search in row-wise and column-wise sorted
- Find the row with maximum number of 1s
- Count pairs
- Sum in matrices
- Median In a Row-Wise sorted Matrix
Related Learning Resources: Matrix Data Structure
Recursion
- Print Pattern
- Handshakes
- Tower of Hanoi
- Josephus problem
- Recursively remove all adjacent duplicates
- Possible words from Phone digits
- Flood fill Algorithm
- Permutations of a string
Related Learning Resources: Recursion
Divide & Conquer Problems
- Write your own power function
- Program for n-th Fibonacci Number
- K-th element of two sorted Arrays
- Median of two sorted arrays
- Karatsuba Algorithm
- The Painter's Partition Problem
- Convex Hull
- Counting inversions
Related Learning Resources: Divide and Conquer Algorithms
Linked List Problems
- Print a Linked List
- Length of a linked list
- Node at a given index in linked list
- Middle of a linked list
- n-th node from end of a linked list
- Delete a node
- Remove every k'th node
- Delete N nodes after M nodes of a linked list
- Delete without head pointer
- Rearrange a linked list
- Segregate even and odd (Using only one traversal)
- Reorder List
- Polynomial Addition
- Insert in a Sorted List
- Swap nodes in pairs
- Reverse a linked list
- Reverse a Linked List in groups of given size.
- Check for palindrome
- Flattening a linked list
- Get intersection point
- Remove duplicates from sorted list
- Remove duplicates from unsorted lists
- Sort a linked list of 0s, 1s and 2s.
- Circular Linked List
- Detect loop in a linked list
- Find length of Loop
- Remove loop in a linked list
- Add two numbers represented by linked lists
- Clone a linked list with random pointers
- Add 1 to a number represented as linked list
- Add two numbers represented as linked list
- Multiply two linked lists
- Merge two sorted linked lists
- Merge Sort on Linked List
- Intersection of Two Linked Lists
- Union of Two Linked Lists
Related Learning Resources: Linked List Data Structure
Doubly and Circular Linked Lists Problems
- Insert a node in Doubly linked list
- Delete node in Doubly Linked List
- Circular Linked List Traversal
- Split a Circular Linked List into two halves
- Insert in Sorted way in a Sorted DLL
- QuickSort on Doubly Linked List
- Merge Sort on Doubly Linked List
- Rotate doubly Linked List by P nodes
- XOR Linked List
Related Learning Resources: Doubly Linked List Circular Linked List
Stack Problems
- Implement Stack using Array
- Implement Stack using Linked List
- Check for balanced parenthesis
- Reverse a stack
- Implement two stacks in an array
- Design a stack with getMin
- The celebrity problem
- Stock Span Problem
- Next Greater Element
- Next Smaller Element
- Longest valid Parentheses
Related Learning Resources: Stack Data Structure
Queue and Dequeue Problems
- Implement Queue using Linked List
- Implement Queue using Array
- Implement Stack using Queue
- Implement Queue using Stack
- Reversing a Queue
- Circular tour
- First non-repeating character in a stream
Related Learning Resources: Queue Data Structure
Prefix Sum and Sliding Window Problems
- Equilibrium Point
- Check if there is a subarray with 0 sun
- Longest Sub-Array with Sum K
- Longest subarray with sum divisible by K
- Largest subarray with equal 1s and 0s
- Longest common span with same number of 1s and 0s among two arrays
- Find mximum sum in any subarray of size k
- Count distinct elements in every window of size k
- Check for subarray with given sum
Related Learning Resources: Prefix Sum and Sliding Window
Bit Magic Problems
- Check if a number is even or odd.
- Number of bit flips
- Game of XOR
- Find bit at a position
- Swap odd and even bits
- Power of 2
- Odd occurring element
- Missing number in array
- Index Of an Extra Element
- Reverse Bits
- Count set bits
- Power Set
Related Learning Resources: Bit Magic
Tree Problems
- Inorder Traversal
- Preorder Traversal
- Postorder Traversal
- Level order traversal
- Find height of Binary Tree
- Count Leaves in Binary Tree
- Check for Children Sum Property
- Mirror Tree
- Check for Balanced Tree
- Lowest Common Ancestor in a Binary Tree
- Diameter of Binary Tree
- Left View of Binary Tree
- Right View of Binary Tree
- Maximum path sum
- Level order traversal line by line
- Tree from Postorder and Inorder
- Tree from Preorder and Inorder
- Connect Nodes at Same Level
- Zig-Zag level order traversal
- Serialize and Deserialize a Binary Tree
- Leaves to DLL
- Binary Tree to Doubly Linked List
- Binary Tree to Circular Doubly Linked List
Related Learning Resources: Tree Data Structure
Binary Search Tree Problems
- BST Search
- BST Insert
- BST Delete
- Minimum in BST
- Inorder Traversal and BST
- Count BST nodes that lie in a given range
- Add all greater values
- Predecessor and Successor in BST
- Closest Neighbor in BST
- Lowest Common Ancestor in a BST
- Convert Level Order Traversal to BST
- Normal BST to Balanced BST
- Pair with given sum in BST
- Check for BST
- Correct BST with two nodes swapped
- Median of BST
- k-th smallest element in BST
- Unique BST's
- Array to BST
- Preorder Traversal and BST
- Preorder to Postorder
- Leaf nodes from preorder traversal
- Triplet with 0 sum in BST
- Merge two BST 's
- Largest BST Subtree
Related Learning Resources: Binary Search Tree
Heap Problems
- Binary Heap Operations
- Height of Heap
- Heap Sort
- Sort a Nearly Sorted Array
- K Largest Elements
- K-th largest element in a stream
- Median of stream
- Merge k sorted arrays
Related Learning Resources: Heap Data Structure
Graph Problems
- Print adjacency list
- Breadth First Search
- Depth First Search
- Find whether path exist
- Knight Walk
- Snake and Ladder Problem
- Bipartite Graph
- Detect Cycle in an undirected graph
- Detect Cycle in a directed graph
- Find first n numbers with given set of digits
- Rotten oranges
- Topological sort
- Shortest Source to Destination Path
- Transitive closure of a Graph
- Strongly Connected Components
Related Learning Resources: Graph Data Structure
Greedy Algorithms Problems
- Fractional Knapsack
- Largest number with given sum
- Activity Selection
- N meetings in one room
- Minimum Platforms
- Minimum number of Coins
- Job Sequencing Problem
- Minimize the heights
- Huffman Coding
- Huffman Decoding
- Minimum Spanning Tree
- Dijkstra for Adjacency Matrix
Related Learning Resources: Greedy Algorithms
Dynamic Programming Problems
- Print first n Fibonacci Numbers.
- Count ways to reach the n’th stair
- Cutted Segments
- Kadane's Algorithm
- Stickler Thief
- Minimum number of jumps
- Total Decoding Messages
- Min Cost Path
- Coin Change
- Longest Common Subsequence
- Consecutive 1's not allowed
- Edit Distance
- Rod Cutting
- Water Overflow
- Maximum Tip Calculator
- Longest Increasing Subsequence
- Maximum sum increasing subsequence
- Max length chain
- 0 - 1 Knapsack Problem
- Interleaved string
- Longest Palindromic Subsequence
- Wildcard Pattern Matching
- Box Stacking
- Longest Bitonic subsequence
- Minimum sum partition
- Largest square formed in a matrix
- Word Break
- Matrix Chain Multiplication
- Special Keyboard
- Egg Dropping Puzzle
- Optimal Strategy for a Game
Related Learning Resources: Dynamic Programming
Backtracking Problems
- Rat Maze With Multiple Jumps
- Coins and Game
- Hamiltonian Path
- Solve the Sudoku
- Combination Sum - Part 2
- Combination Sum
- Subsets
- Largest number in K swaps
- M-Coloring Problem
- Black and White
Related Learning Resources: Backtracking
Trie Problems
- Trie Search and Insert
- Trie Delete
- Unique rows in a binary matrix
- Count of distinct substrings
- Word Boggle
Related Learning Resources: Trie Data Structure
Practice Questions to Test Your Overall Learning
- Longest common prefix
- Implement Atoi
- Two numbers with sum closest to zero
- Smallest greater elements in whole array
- Max rectangle
- Find triplets with zero sum
- Counting elements in two arrays
- Merge K sorted linked lists
- Maximum Difference
- Circle of strings
- All possible Word Breaks
- Alien Dictionary
- Design a tiny URL or URL shortener
- Implement LRU Cache
Whiteboard Challenges and Independent Coding Interview Questions
In many coding interviews, especially during college recruitment, you’ll encounter whiteboard challenges or independent coding tests. These tests are designed to assess how well you can solve problems without relying on an IDE. You need to demonstrate your ability to write code, handle errors, and optimize your solution without real-time debugging.
What are whiteboard challenges?
In a whiteboard challenge, you’re typically given a coding problem and asked to solve it on a whiteboard in front of the interviewer. This can feel intimidating because you’re writing without the help of an IDE, but it’s an excellent opportunity to showcase your problem-solving approach. In independent coding tests, the focus remains on solving coding problems, but in an online setting, you're expected to write fully functional code.
Example Questions
Here are some example questions that you can get in a whiteboard challenge or independent coding test:
- How do you reverse a string?
- How do you determine if a string is a palindrome?
- How do you count the occurrences of each character in a string?
- How to check unique characters in a string?
- How do you find out if the two given strings are anagrams?
- How do you reverse an array?
- How do you find the maximum element in an array?
- How do you sort an array of integers in ascending order?
- How to get an nth number in a Fibonacci sequence using recursion?
- How do you calculate the sum of two integers?
- How do you find the average of numbers in a list?
- How do you check if an integer is even or odd?
- How do you find the middle element of a linked list?
- How do you remove a loop in a linked list?
- How do you merge two sorted linked lists?
- What is a binary search?
- What is the difference between a binary tree and a binary search tree?
- How do you print a binary tree in vertical order?
Tips to Prepare for Coding Interviews
Just knowing these coding questions isn’t enough. Let’s see how you should approach preparation.
- Consistency is key: Practice coding questions every day. You don’t need to solve a dozen problems at once-just focus on 1-2 high-quality problems each day and make sure you understand them fully. You may explore our must-know coding problems cheatsheet to ensure you cover the most commonly asked questions in coding interviews.
- Understand the problem, don’t memorize: Memorizing solutions won’t help in a real interview. Make sure you understand the underlying logic behind each problem.
- Choose One Programming Language and Master It: A single language at a time is easy to master.
- Do Mock Interviews: Practice with friends, mentors, or online platforms.
- Explain Your Thinking Out Loud: During interviews, clearly explain how you approach the problem.
- Assess Common Coding Questions: Study the most common questions about coding.
- Understand the Job Role and Company: Carefully read the job description and research the company.
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