For Students

Interview Prep

Free resources for the topics our technical interviews cover. Interview in Java or Python, whichever you prefer.

Object-Oriented Fundamentals

Be ready to explain each of the four pillars in your own words, or to walk through a program you have written and show where you used inheritance, polymorphism, encapsulation and abstraction.

The four pillars

A 30-minute overview of all four ideas, then each language's official introduction to classes and objects.

Intro to Object Oriented Programming - Crash Course (video) freeCodeCamp

Java

Object-Oriented Programming Concepts Oracle Java Tutorials

Python

Classes Python documentation

Inheritance

A subclass reuses the fields and methods of its parent class and adds only what is different.

Java

Inheritance Dev.java

Python

Class hierarchies University of Helsinki MOOC.fi · Free account only for the exercises

Polymorphism

One method call, different behaviour: each object answers the same call in its own way, decided at run time.

Java

Java Programming II: Object polymorphism University of Helsinki MOOC.fi

Python

Class hierarchies (overriding methods) University of Helsinki MOOC.fi · Free account only for the exercises

Encapsulation

An object keeps its data private and lets other code change it only through methods that can check the new value.

Java

Controlling Access to Members of a Class Oracle Java Tutorials

Python

Access modifiers University of Helsinki MOOC.fi · Free account only for the exercises

Abstraction

Code depends on what an object can do, not on how it does it: abstract classes and interfaces declare the behaviour, subclasses supply it.

Java

Abstract Methods and Classes Oracle Java Tutorials

Python

abc: Abstract Base Classes Python documentation

Strings & Hash Maps

If you are selected for a follow-up interview, you may be asked to complete a short coding problem that builds, counts or compares strings, usually with a hash map and sometimes a sliding window.

Strings

Walking a string character by character, building a new one efficiently, and the built-in methods you will reach for.

Java

StringBuilder Java SE 21 API

Python

String Methods Python documentation

Hash maps

Count, group and look things up in constant time: the tool behind most string problems.

Java

Java Programming I: Hash Map University of Helsinki MOOC.fi

Python

collections.Counter Python documentation

Sliding window

Move two indexes along a string or array instead of re-checking every substring.

Sliding Window Technique GeeksforGeeks · Examples in Java, Python and more

Practice Problems

Free public problems similar to what we ask. Solve them in Java or Python; a free LeetCode account is needed only to submit a solution.

Easy 387. First Unique Character in a String
Easy 242. Valid Anagram
Medium 49. Group Anagrams
Medium 3. Longest Substring Without Repeating Characters
Medium 443. String Compression
Similar to Cracking the Coding Interview 1.6. LeetCode edits the array in place and leaves out the count for a single character; the classic version returns a new string with every count.

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