Coding interview questions

Check if a string is a palindrome (ignore case, alphanumeric)

Easy · Coding, Strings, Java 17+

Validate balanced parentheses using a stack

Easy · Coding, Strings, Java 17+

Reverse word order in a sentence in-place style

Easy · Coding, Strings, Java 17+

Longest substring without repeating characters

Medium · Coding, Strings, Java 17+

Compress string with run-length encoding

Medium · Coding, Strings, Java 17+

Count vowels and consonants in a string

Easy · Coding, Strings, Java 17+

Check if s2 is a rotation of s1

Easy · Coding, Strings, Java 17+

Find longest common prefix among strings

Easy · Coding, Strings, Java 17+

Remove all adjacent duplicate characters recursively

Medium · Coding, Strings, Java 17+

Implement atoi string to integer with overflow handling

Hard · Coding, Strings, Java 17+

Two Sum using HashMap

Easy · Coding, Arrays, Java 17+

Rotate array to the right by k steps

Medium · Coding, Arrays, Java 17+

Find missing number in 1..n array

Easy · Coding, Arrays, Java 17+

Find duplicate in array of n+1 integers

Medium · Coding, Arrays, Java 17+

Merge two sorted arrays into one sorted array

Easy · Coding, Arrays, Java 17+

Move all zeros to end while preserving order

Easy · Coding, Arrays, Java 17+

Product of array except self without division

Medium · Coding, Arrays, Java 17+

Maximum subarray sum (Kadane's algorithm)

Medium · Coding, Arrays, Java 17+

Binary search on sorted array

Easy · Coding, Arrays, Java 17+

Intersection of two integer arrays

Easy · Coding, Arrays, Java 17+

Find pair with given sum in unsorted array

Easy · Coding, Arrays, Java 17+

Search insert position in sorted array

Easy · Coding, Arrays, Java 17+

Reverse a String using Streams.

Easy · Coding, Streams, Java 17+

Check if two strings are anagrams using Streams.

Easy · Coding, Streams, Fresher

Find the first non-repeating character in a String using Java 8 Streams.

Medium · Coding, Streams, Senior

Find the frequency of each character in a string using HashMap and Streams.

Easy · Coding, Streams, Fresher

Count occurrences of each word in a sentence (word frequency map).

Easy · Coding, Streams, Fresher

Find duplicate elements in a list using Streams.

Easy · Coding, Streams, Fresher

Remove duplicates from a list using Java 8.

Easy · Coding, Streams, Fresher

Check if a list contains only unique elements using Streams.

Easy · Coding, Streams, Fresher

Find the maximum and minimum element in a list.

Easy · Coding, Streams, Fresher

Find the second highest/lowest element in a list.

Easy · Coding, Streams, Fresher

Find the top N elements in a list using Streams.

Easy · Coding, Streams, Fresher

Sort a Map by values using Streams.

Easy · Coding, Streams, Fresher

Find common elements between two lists using Streams.

Easy · Coding, Streams, Fresher

Flatten a list of lists using flatMap.

Easy · Coding, Streams, Fresher

Merge multiple lists into one using Streams.

Easy · Coding, Streams, Fresher

Convert List<Integer> to comma-separated String using Collectors.

Easy · Coding, Streams, Fresher

Partition numbers into even/odd using Collectors.partitioningBy.

Easy · Coding, Streams, Fresher

Group employees by department using Collectors.groupingBy.

Medium · Coding, Streams, Senior

Find the average salary of employees using Streams.

Easy · Coding, Streams, Fresher

Find the longest string in a list using reduce().

Easy · Coding, Streams, Fresher

Count the number of words in a file using parallelStream().

Medium · Coding, Streams, Concurrency

Convert a list into a Map using Collectors.toMap().

Medium · Coding, Streams, Senior

Custom sort an Employee list by salary and then by name.

Medium · Coding, Collections, Java 8+

Implement a custom Comparator with multiple fields (department, salary descending, name).

Medium · Coding, Collections, Java 8+

Implement an LRU Cache using LinkedHashMap.

Hard · Coding, Collections, Architect

Design a bounded blocking queue from scratch.

Hard · Coding, Collections, Concurrency

Sort map entries by frequency descending.

Medium · Coding, Collections, Senior

Top K frequent elements using PriorityQueue.

Medium · Coding, Collections, Senior

Range query on TreeMap using subMap.

Medium · Coding, Collections, Senior

Create a defensive immutable copy of a list (Java 17+).

Easy · Coding, Collections, Java 17+

Deque-based sliding window maximum.

Hard · Coding, Collections, Staff

Thread-safe iteration with CopyOnWriteArrayList demo.

Medium · Coding, Collections, Concurrency

Implement a stack using an array.

Easy · Coding, Data Structures, Stack

Implement a queue using two stacks.

Medium · Coding, Data Structures, Queue

Reverse a singly linked list iteratively.

Easy · Coding, Data Structures, Linked List

Detect a cycle in a linked list (Floyd's algorithm).

Medium · Coding, Data Structures, Linked List

Merge two sorted linked lists.

Medium · Coding, Data Structures, Linked List

Validate a binary search tree.

Medium · Coding, Data Structures, Binary Tree

Binary tree level-order traversal (BFS).

Medium · Coding, Data Structures, Binary Tree

Invert a binary tree recursively.

Easy · Coding, Data Structures, Binary Tree

Find the kth largest element using a min-heap.

Medium · Coding, Data Structures, Heap

Design a HashMap with separate chaining.

Hard · Coding, Data Structures, HashMap

Compute Fibonacci numbers with memoization.

Easy · Coding, Recursion, DP

Compute factorial iteratively and recursively.

Easy · Coding, Recursion, Fresher

Generate all permutations of an array.

Medium · Coding, Recursion, Backtracking

Generate the power set via backtracking.

Medium · Coding, Recursion, Backtracking

Solve the Tower of Hanoi puzzle.

Medium · Coding, Recursion, Senior

Climbing stairs — count distinct ways (DP).

Easy · Coding, Recursion, DP

Coin change — minimum number of coins (DP).

Hard · Coding, Recursion, DP

N-Queens — count the number of solutions.

Hard · Coding, Recursion, Backtracking

Implement merge sort (divide and conquer).

Medium · Coding, Algorithms, Sorting

Implement quicksort with Lomuto partition.

Medium · Coding, Algorithms, Sorting

Sliding window maximum.

Hard · Coding, Algorithms, Deque

Longest increasing subsequence length in O(n log n).

Hard · Coding, Algorithms, DP

Topological sort using Kahn's algorithm.

Hard · Coding, Algorithms, Graph

BFS shortest path in an unweighted graph.

Medium · Coding, Algorithms, Graph

DFS cycle detection in a directed graph.

Medium · Coding, Algorithms, Graph

Sort colors — Dutch national flag algorithm.

Medium · Coding, Algorithms, Array

Implement a producer-consumer pattern using BlockingQueue.

Medium · Coding, Concurrency, Collections

Print odd and even numbers alternately using two threads.

Medium · Coding, Concurrency, Senior

Demonstrate asynchronous computation using CompletableFuture.

Medium · Coding, Concurrency, Java 8+

Implement a thread-safe Singleton using Bill Pugh and Double-Checked Locking patterns.

Hard · Coding, Concurrency, Design Principles

Implement a barrier for 3 threads using CyclicBarrier.

Hard · Coding, Concurrency, Staff

Write a program where 3 threads print A, B, C in sequence repeatedly.

Hard · Coding, Concurrency, Staff

Implement a multithreaded word counter using ConcurrentHashMap.

Hard · Coding, Concurrency, Collections

ExecutorService fixed thread pool task submission.

Medium · Coding, Concurrency, Senior

ReadWriteLock for cache read-heavy workload.

Hard · Coding, Concurrency, Architect

CountDownLatch service startup coordinator.

Medium · Coding, Concurrency, Senior

Semaphore-based rate limiter.

Hard · Coding, Concurrency, Architect

Virtual threads (Java 21) for concurrent HTTP-style tasks.

Medium · Coding, Concurrency, Java 21

Builder pattern for an immutable object.

Medium · Coding, OOP & Patterns, Builder

Strategy pattern for sorting comparators.

Medium · Coding, OOP & Patterns, Strategy

Factory method pattern.

Medium · Coding, OOP & Patterns, Factory Method

Observer pattern — event bus.

Medium · Coding, OOP & Patterns, Observer

Java 17 record as a DTO with validation.

Easy · Coding, OOP & Patterns, Records

Sealed classes with pattern matching.

Medium · Coding, OOP & Patterns, Sealed Classes

Template method for data export.

Medium · Coding, OOP & Patterns, Template Method

Decorator pattern — coffee pricing.

Medium · Coding, OOP & Patterns, Decorator

Check if two strings are anagrams (character frequency)

Easy · Coding, Strings, HashMap

Group anagrams from a list of strings

Easy · Coding, Strings, HashMap

Find all unique triplets that sum to zero (3Sum)

Medium · Coding, Strings, Two Pointers

Minimum window substring containing all characters of t

Hard · Coding, Strings, Sliding Window

Find the longest palindromic substring

Medium · Coding, Strings, Two Pointers

Container with most water (two pointers)

Medium · Coding, Arrays, Two Pointers

Trapping rain water between elevation bars

Hard · Coding, Arrays, Two Pointers

Count subarrays with sum equal to k (prefix sum + HashMap)

Medium · Coding, Arrays, Prefix Sum

Count number of islands in a 2D grid (DFS/BFS)

Medium · Coding, Graphs, DFS

Number of connected components using Union-Find

Medium · Coding, Graphs, Union-Find

Dijkstra's shortest path in a weighted graph

Hard · Coding, Graphs, Graph

Word Search — find word in 2D grid (backtracking)

Medium · Coding, Graphs, Backtracking

Alien Dictionary — derive character order from sorted words (topo sort)

Hard · Coding, Graphs, Graph

Word Ladder — shortest transformation sequence (BFS)

Hard · Coding, Graphs, BFS

House Robber — maximum loot without adjacent houses

Easy · Coding, Dynamic Programming, DP

Longest Common Subsequence (LCS)

Medium · Coding, Dynamic Programming, DP

0/1 Knapsack — maximum value with weight limit

Medium · Coding, Dynamic Programming, DP

Edit Distance (Levenshtein) between two strings

Hard · Coding, Dynamic Programming, DP

Decode Ways — count ways to decode a digit string (1-26 → A-Z)

Medium · Coding, Dynamic Programming, DP

Merge overlapping intervals

Medium · Coding, Intervals, Sorting

Non-overlapping intervals — minimum removals to eliminate overlap

Medium · Coding, Intervals, Greedy

Meeting Rooms II — minimum conference rooms required

Medium · Coding, Intervals, Heap

Find the single number (every other appears twice) using XOR

Easy · Coding, Bit Manipulation, Fresher

Count set bits (Hamming weight) in an integer

Easy · Coding, Bit Manipulation, Fresher

Check if a number is a power of two

Easy · Coding, Bit Manipulation, Fresher

Search in rotated sorted array

Medium · Coding, Binary Search, Senior

Koko Eating Bananas — binary search on answer

Medium · Coding, Binary Search, Staff

Jump Game II — minimum jumps to reach last index

Medium · Coding, Greedy, BFS

Jump Game I — can you reach the last index?

Medium · Coding, Greedy, Senior

Activity Selection — maximum non-overlapping intervals

Medium · Coding, Greedy, Intervals

Next Greater Element for each array position

Medium · Coding, Monotonic Stack, Stack

Daily Temperatures — days until warmer temperature

Medium · Coding, Monotonic Stack, Stack

Largest rectangle in histogram

Hard · Coding, Monotonic Stack, Stack

Spiral matrix traversal

Medium · Coding, Matrix, Senior

Validate a 9×9 Sudoku board

Medium · Coding, Matrix, HashSet

Set matrix zeroes in-place

Medium · Coding, Matrix, Senior

Implement Trie (Prefix Tree) with insert, search, startsWith

Medium · Coding, Trie, Senior

Word Search II — find all words from dictionary in grid (Trie + DFS)

Hard · Coding, Trie, Backtracking

Maximum depth of a binary tree

Easy · Coding, Data Structures, Binary Tree

Lowest Common Ancestor in a BST

Easy · Coding, Data Structures, BST

Serialize and deserialize a binary tree

Hard · Coding, Data Structures, Binary Tree

Diameter of a binary tree (longest path between any two nodes)

Medium · Coding, Data Structures, Binary Tree

Merge k sorted linked lists

Hard · Coding, Data Structures, Heap

Find median from a data stream (two heaps)

Hard · Coding, Data Structures, Heap

Root-to-leaf path sum equals target

Easy · Coding, Data Structures, Binary Tree

Design Hit Counter — count hits in past 5 minutes

Medium · Coding, System Design Coding, Queue

Design Twitter feed — merge k sorted post streams

Hard · Coding, System Design Coding, Heap

Consistent hashing — locate key on a server ring

Hard · Coding, System Design Coding, Distributed Systems

Distributed unique ID generator (Snowflake-style)

Hard · Coding, System Design Coding, Distributed Systems

Idempotent API handler with idempotency keys

Medium · Coding, System Design Coding, Design

Circuit breaker pattern implementation

Medium · Coding, System Design Coding, Design

Platform plugin registry — load and invoke extensions safely

Hard · Coding, System Design Coding, Platform

Schema registry — validate and version event payloads

Hard · Coding, System Design Coding, Platform

Policy engine — evaluate allow/deny rules on requests

Hard · Coding, System Design Coding, Platform

Median of two sorted arrays (binary partition)

Hard · Coding, System Design Coding, Binary Search

Saga orchestration — coordinate distributed transactions

Hard · Coding, System Design Coding, Distributed Systems