📚 Based on Real Interview Questions

Cracking the
Coding Interview
Top Java Challenges

50+ real interview challenges solved step-by-step in Java. Master the data structures, algorithms, and problem-solving patterns that top companies actually test.

50+
Challenges
10
Topics
100%
Java Code
⭐ 4.8
Rating
🛒 Buy on Amazon Read Free Sample

CRACKING THE CODING INTERVIEW

TOP JAVA CHALLENGES  •  Moises Gamio

Big O Notation order of growth chart

by Moises Gamio — codersite.dev

Curriculum

What You'll Master

Every topic is explained from first principles, coded in Java, and verified with real unit tests — the way interviewers expect you to think.

⚙️

Big O Notation & Complexity

Analyze time and space complexity across O(1), O(N), O(N²), O(log N), and beyond. Know which algorithm wins and exactly why.

Foundation
📌

Arrays & Strings

Reverse text, detect palindromes, rotate matrices, compare version numbers, and solve real-world locker location problems.

8 Challenges
🔗

Linked Lists

Implement, reverse, and deduplicate linked lists. Design a full LRU cache from scratch with O(1) get and put.

4 Challenges
🧮

Math & Logic Puzzles

FizzBuzz, prime validation, power-of-base checks, immutable currency conversion, and minimum assembly time optimization.

10 Challenges
🔄

Recursion

Factorial, Fibonacci, full permutation generation, and GCD — all reasoned from base case to recursive call stack.

4 Challenges
📈

Sorting & Searching

Bubble, Insertion, Quick Sort. Binary Search. Merge sorted lists. Find missing arithmetic progression terms.

7 Challenges
💾

Stacks & Queues

Delimiter matching, implementing a FIFO queue via two stacks, reversing strings using a stack data structure.

3 Challenges
🔑

Hash Tables

Design a hash table from scratch. Find the most frequent elements in an array. Solve the Nuts and Bolts pairing problem.

3 Challenges
🌳

Trees & Graphs

Binary Search Trees, DFS, BFS, in-order/pre-order traversal, and a full social network graph traversal algorithm.

5 Challenges
🏆

Real Coding Challenges

Build Tic-Tac-Toe, optimize online purchases, simulate a car filling station queue, and design a complete bank account system.

5 Full Projects
Free Extract

Get a Feel for the Book

Here is a real solved problem from Chapter 1 — exactly how each challenge is presented: clear problem statement, efficient solution, visual reasoning, complexity analysis, and unit tests.

Challenge 1.3 — Validate If a String Is a Palindrome

A palindrome reads the same forward and backward (e.g., level, madam). The solution loops from both ends inward and exits early on any mismatch.

✓ O(N) time    ✓ O(1) space    ✓ Tested with JUnit

Read More on Amazon →
StringUtils.java
// Listing 1.3 – Validate If a String Is a Palindrome public class StringUtils { public static boolean isPalindrome(String text) { final int length = text.length(); for (int idx = 0; idx < length / 2; idx++) { if (text.charAt(idx) != text.charAt(length - 1 - idx)) return false; } return true; } }
IsPalindromeTest.java
@Test public void is_palindrome() { assertTrue(StringUtils.isPalindrome("madam")); assertTrue(StringUtils.isPalindrome("99")); assertTrue(StringUtils.isPalindrome("2f1f2")); } @Test public void is_not_palindrome() { assertFalse(StringUtils.isPalindrome("2f1")); assertFalse(StringUtils.isPalindrome("-101")); }
Complexity Analysis: Time O(N) — each character is visited at most once. Space O(1) — no extra data structures needed. The early-exit strategy means the algorithm stops at the first mismatch, making it highly efficient on large inputs.
Audience

Who This Book Is For

Whether you're preparing for your first junior role or chasing a senior position at a top tech company, this book meets you where you are.

🎓

Junior & Mid-Level Developers

Solidify your Java fundamentals and data structures knowledge before stepping into technical interviews at any company size.

💼

Career Switchers

Coming from another language or field? Step-by-step explanations and visual diagrams get you interview-ready fast.

🚀

Senior Engineers

Sharpen your algorithm thinking and refresh the classic patterns that interviewers at startups and big tech rely on.

🏫

CS Students

Bridge the gap between university theory and what companies actually ask. Every solution includes working, tested Java code.

Reviews

What Engineers Are Saying

Developers worldwide used this book to land their next role. Here is what they found most valuable.

“

Finally a Java-specific interview book that doesn't just copy LeetCode. The step-by-step explanations and actual unit tests made me understand why each solution works, not just how to memorize it.

⭐⭐⭐⭐⭐
AM
Alex M.
Backend Engineer — Berlin
“

I used this book to prepare for interviews at two fintech companies. The Big O notation section alone saved me during the technical screen. Got both offers and negotiated a 23% salary increase.

⭐⭐⭐⭐⭐
SR
Sofia R.
Java Developer — Madrid
“

The diagrams are excellent. I had been coding for 5 years and still had gaps in my understanding of linked lists and trees. This book closed those gaps in a single weekend.

⭐⭐⭐⭐⭐
DK
David K.
Senior Software Engineer — Warsaw
“

The real-world coding challenges at the end — Tic-Tac-Toe, bank account, car station simulation — are brilliant. They test design thinking, not just algorithm recall. Perfect for senior-level rounds.

⭐⭐⭐⭐⭐
LT
Laura T.
Tech Lead — Amsterdam
“

As someone transitioning from Python to Java, this was a lifesaver. Everything is explained from scratch without assuming prior Java knowledge, yet the code is production quality.

⭐⭐⭐⭐★
JP
James P.
Software Engineer — London
“

I passed my interview at a logistics company after studying the arrays and graphs chapters. The DHL Locker and Social Network Traversal problems are exactly the real-world scenarios companies now use.

⭐⭐⭐⭐⭐
MV
Maria V.
Junior Developer — Buenos Aires
“

A MUST read for any Java Developer! Succinct and super informative!. The author tries his best to keep the explanation as succinct as possible at the same time providing the code snippets and explanations including pictorial representation of problem-solutions!.

⭐⭐⭐⭐⭐
MK
Madhusudhan Konda
AI Strategy & Engineering Leader — London
About the Author
MG
codersite.dev

Moises Gamio

Senior Java Developer & Software Engineer — Founder of codersite.dev

Moises Gamio is a software engineer who has sat on both sides of the interview table — as a candidate at startups and large corporations, and as a technical interviewer. Over the years he collected, refined, and documented real interview questions that were actually asked in the field. That lived experience became this book.

Founder of codersite.dev, Moises writes about Java, software design, and algorithmic thinking to help developers worldwide land better roles, write better code, and negotiate better salaries. He is also the author of Software Design Principles: A Practical Guide.

🌐 codersite.dev 📚 Amazon Author Page ✉ Contact

Your Next Job Offer Starts Here

Join thousands of Java developers who prepared smarter, interviewed with confidence, and landed the role they wanted.

🛒 Buy on Amazon — Paperback 📱 Kindle Edition

Available in paperback and Kindle  •  ISBN: 9798650252368  •  Ships worldwide