Interviews — User Guide

1. Quick Start

Interviews is a comprehensive collection of Java solutions for coding interview preparation. It covers LeetCode problems (organized by algorithm category), company-specific interview questions from top tech firms, Cracking the Coding Interview (CTCI) chapter exercises, and UVA Online Judge problems.

Key features:

• 500+ Java solutions spanning algorithms, data structures, and problem-solving patterns
• LeetCode problems organized by 21 algorithm categories (arrays, dynamic programming, trees, graphs, and more)
• Company-tagged questions for 15 major tech companies including Google, Facebook, Amazon, and Microsoft
• CTCI chapter exercises covering 7 core topic areas
• 47 UVA Online Judge competitive programming solutions
• Every solution file includes the problem statement as comments
• Clean, readable Java implementations with inline explanations

Prerequisites: Git and Java JDK 8 or later.

git clone https://github.com/kdn251/interviews.git
cd interviews

Open any solution file to study the problem and its implementation:

cat leetcode/array/BestTimeToBuyAndSellStock.java

// Say you have an array for which the ith element is the price of a given stock on day i.
// If you were only permitted to complete at most one transaction ...
public class BestTimeToBuyAndSellStock {
    public int maxProfit(int[] prices) {
        //Kadane's algorithm
        ...
    }
}

Tip

Start with leetcode/array/ — array problems are the most common interview topic and the solutions here cover 29 distinct problems ranging from easy to hard.

What to try next: Browse company-specific problems in company/google/ or company/facebook/, or study algorithm patterns in leetcode/dynamic-programming/.

2. Installation

System Requirements

Getting the Repository

HTTPS

git clone https://github.com/kdn251/interviews.git

SSH

git clone git@github.com:kdn251/interviews.git

GitHub CLI

gh repo clone kdn251/interviews

Verifying Your Setup

Compile any solution to confirm Java is working:

cd interviews/leetcode/array
javac BestTimeToBuyAndSellStock.java

(no output on success)

Important

Most solution files are standalone classes without a main method — they are designed as algorithm implementations, not runnable programs. To test them, write a small driver class or use an IDE's built-in test runner. The UVA solutions are the exception: they include main methods with Scanner-based I/O for online judge submission.

IDE Setup (Optional)

The repository includes IntelliJ IDEA project files (interviews.iml, LeetCode.iml, CrackingTheCodingInterview.iml, Company.iml). To open in IntelliJ:

1. Open IntelliJ IDEA
2. Select File > Open
3. Navigate to the cloned interviews/ directory
4. Click OK — IntelliJ will detect the .iml files automatically

For VS Code, install the Extension Pack for Java and open the folder directly.

Note

The .iml project files define IntelliJ module boundaries for the three main sections (LeetCode, Company, CTCI). They are not required for compilation — any Java-capable editor works.

3. Core Usage

Understanding the Repository Structure

Solutions are organized into four top-level collections:

Browsing by Algorithm Category (LeetCode)

LeetCode solutions are grouped into 21 algorithm categories. Each file is named after the problem (e.g., TwoSum.java, MergeIntervals.java) and contains:

1. The full problem statement as comments at the top
2. A clean Java solution class
3. Inline comments explaining the approach

Tip

Many problems appear in multiple categories because they can be solved with different techniques. For example, CountingBits.java appears in both bit-manipulation/ and dynamic-programming/ — comparing both teaches you when to pick one pattern over another.

Browsing by Company

The company/ directory groups problems by the tech company known to ask them in interviews:

Note

The same problem often appears under multiple companies. For instance, TwoSum.java exists in company/facebook/, company/amazon/, company/uber/, company/linkedin/, and company/airbnb/. Each copy is the same implementation — the duplication reflects that multiple companies ask the same question.

Browsing by CTCI Chapter

The cracking-the-coding-interview/ directory follows the book's chapter structure:

Browsing UVA Online Judge Solutions

The uva/ directory contains 47 solutions for competitive programming problems from the UVA Online Judge. Unlike the LeetCode and company solutions, these include main methods with Scanner-based standard I/O, ready for direct submission.

// Example structure: uva/AddingReversedNumbers.java
import java.math.BigInteger;
import java.util.Scanner;

public class AddingReversedNumbers {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int numberOfTestCases = input.nextInt();
        // ...processes input and prints reversed sum
    }
}

Warning

UVA solutions use java.util.Scanner for console input. When running them locally, you must provide input via stdin (pipe a file or type interactively). They will hang if launched without input.

4. Common Tasks

Study a Specific Algorithm Pattern

Goal: Master a single algorithm technique (e.g., dynamic programming).

1. Navigate to the category folder:

   ls leetcode/dynamic-programming/
   

2. Start with simpler problems: ClimbingStairs.java, HouseRobber.java, CoinChange.java
3. Read the problem statement (top comments) before looking at the solution
4. Study the solution's approach — note the recurrence relation and base cases
5. Progress to harder problems: EditDistance.java, RegularExpressionMatching.java

Prepare for a Company Interview

Goal: Review the most commonly asked questions for a specific company.

1. List problems for your target company:

   ls company/facebook/
   

   3Sum.java                              MergeKSortedLists.java
   AddAndSearchWordDataStructureDesign.java  MergeSortedArray.java
   AddBinary.java                         MinStack.java
   ...65 files total
   

2. Cross-reference with the LeetCode category folders to understand which algorithm each problem tests
3. Solve the problems independently, then compare your solution with the repository's approach
4. Focus on the companies with the most problems (Facebook: 65, Google: 60, Amazon: 27) for the broadest coverage

Work Through CTCI Systematically

Goal: Follow the Cracking the Coding Interview book chapter by chapter.

1. Open the book to the relevant chapter
2. Navigate to the matching directory:

   ls cracking-the-coding-interview/chapter-one-arrays-and-strings/
   

   DeleteDups.java    IsUniqueChars.java  Permutation.java
   IsRotation.java    NthToLast.java      ReplaceSpaces.java
   

3. Attempt each problem on paper or in your IDE before reading the solution
4. Compare your approach with the repository's implementation
5. Note the techniques used — for example, IsUniqueChars.java uses an int as a 26-bit boolean array via bit manipulation

Compare Multiple Approaches to the Same Problem

Goal: See how the same problem is solved differently across categories.

Some problems appear in multiple LeetCode category folders. To find all instances of a problem:

find . -name "TwoSum.java"

./company/airbnb/TwoSum.java
./company/amazon/TwoSum.java
./company/facebook/TwoSum.java
./company/linkedin/TwoSum.java
./company/uber/TwoSum.java
./leetcode/hash-table/TwoSum.java

Tip

When a problem appears in both a LeetCode category folder and a company folder, the implementations are typically identical. The LeetCode category tells you the technique (hash table), while the company tag tells you where it is asked (Facebook, Amazon, etc.).

Compile and Test a Solution

Goal: Run a solution locally to verify it or experiment with modifications.

Most solutions lack a main method. Create a simple test driver:

// TestDriver.java (create in same directory as the solution)
public class TestDriver {
    public static void main(String[] args) {
        BestTimeToBuyAndSellStock solution = new BestTimeToBuyAndSellStock();
        int[] prices = {7, 1, 5, 3, 6, 4};
        System.out.println(solution.maxProfit(prices)); // Expected: 5
    }
}

javac BestTimeToBuyAndSellStock.java TestDriver.java
java TestDriver

5

Study Data Structure Implementations

Goal: Learn how data structures are implemented from scratch.

Several solutions implement custom data structures:

Identify the Most-Asked Problems Across Companies

Goal: Find high-frequency problems that appear at many companies.

These problems appear in 4 or more company folders:

5. Troubleshooting

Error: cannot find symbol

Cause: Solution files reference standard library classes (HashMap, ArrayList, LinkedList) without explicit imports. The files are written as algorithm exercises, not complete compilation units.

Solution: Add the necessary imports at the top of the file:

import java.util.*;

Prevention: When creating test drivers, always include import java.util.*; at the top.

Error: class name doesn't match file name

Cause: A few files have class names that differ from the filename. For example, cracking-the-coding-interview/chapter-one-arrays-and-strings/DeleteDups.java contains class RemoveDups instead of class DeleteDups.

Solution: Either rename the class to match the file, or rename the file to match the class.

Warning

The file company/google/RegularExpressionMatching.javaa has a double-a extension (.javaa instead of .java). Rename it to compile: mv RegularExpressionMatching.javaa RegularExpressionMatching.java.

Error: cannot access Scanner (UVA solutions)

Cause: UVA solutions use java.util.Scanner for stdin input. Running them without providing input causes the program to hang waiting for console input.

Solution: Provide input via a file or pipe:

echo -e "3\n24 1\n4358 754\n305 794" | java AddingReversedNumbers

34
1998
1

Files won't compile due to missing type definitions

Cause: Some solutions reference helper types like LinkedListNode, TreeNode, or ListNode that are not defined in the repository. These are standard LeetCode platform-provided types.

Solution: Define the missing class locally:

// For linked list problems
class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

// For tree problems
class TreeNode {
    int val;
    TreeNode left, right;
    TreeNode(int x) { val = x; }
}

Prevention: Keep a Common.java file with these definitions and compile it alongside any solution that needs them.

Duplicate files across folders — which one to study?

Cause: The same problem (e.g., TwoSum.java) exists in multiple company folders and in a LeetCode category folder. The implementations are typically identical.

Solution: Study the LeetCode category version — the folder name tells you the algorithm technique. The company folder copies exist to map problems to interview contexts.

Caution

Do not assume every copy is identical. A small number of problems have minor implementation differences between the LeetCode and company versions. If you find a discrepancy, the LeetCode category version is generally the more polished implementation.

6. FAQ

Q: What Java version do I need?

Java 8 or later. The solutions use standard library collections (HashMap, ArrayList, Arrays), lambda-free iteration patterns, and no module system features. Any JDK from 8 onward compiles everything.

Q: Are these solutions optimal?

The solutions demonstrate clean, correct approaches but are not always the most space- or time-efficient possible. They prioritize readability and teaching the core algorithm pattern. For example, FindAllNumbersDisappearedInAnArray.java uses a HashMap where an in-place marking approach would use O(1) extra space.

Q: Why are there duplicate files across company folders?

The repository mirrors how interview prep works in practice — you study by target company. If Facebook, Google, and Amazon all ask "Merge K Sorted Lists," the file appears in all three company folders. This makes it easy to get a complete list of problems for any single company without cross-referencing.

Q: Which section should I start with?

For general interview prep, start with leetcode/array/ and leetcode/string/ — these are the most common categories. For targeted company prep, go directly to company/{your_target}/. For a structured curriculum, follow the CTCI chapters in order.

Q: Can I submit the UVA solutions directly to the online judge?

Yes. The UVA solutions include main methods with Scanner-based I/O matching the judge's expected format. Copy the file contents into the submission form. LeetCode and company solutions need the platform-specific Solution class wrapper that LeetCode provides.

Q: Is there a build system (Maven, Gradle)?

No. The repository has no build system, no pom.xml, no build.gradle. Each .java file is a standalone compilation unit. Compile individual files with javac or open the project in an IDE.

Q: How do I contribute new solutions?

Place your solution in the appropriate category folder, name the file after the problem (PascalCase), include the problem statement as comments at the top, and submit a pull request. See the Admin Guide for the full repository structure and coding conventions.

Q: Why are some CTCI chapters missing (6, 8, 10+)?

The repository covers chapters 1-5, 7, and 9. Chapters 6, 8, 10, and later are not yet implemented. GitHub issue #211 discusses renaming and expanding chapter coverage. Issue #144 requests more Adobe problems.

Q: Are there any known content issues?

GitHub issue #200 reports a misleading example in the Greedy Algorithms section. Issue #261 notes a duplication in the Queue data structure section. Check the issue tracker for current status.