UIUC CS 225: Data Structures
CS 225 is UIUC's famous data structures course in C++, covering lists, trees, AVL and B-trees, hashing, heaps, disjoint sets, and graph algorithms, delivered through MPs (machine problems) and weekly labs. It's the make-or-break course of the CS major and the one alumni still talk about.
Fennie is independent and not affiliated with University of Illinois Urbana-Champaign. This is an unofficial study guide.
What makes it hard
The MPs are substantial C++ projects with autograders and tight specs, and they run alongside theory exams in the computer-based testing facility, so you run two demanding tracks at once. The C++ is real (memory management, templates, iterators), and students whose 128 foundations are shaky pay for it here.
What you'll cover
- • C++ classes, memory, and templates
- • Linked lists and trees
- • AVL trees and B-trees
- • Hash tables
- • Heaps and priority queues
- • Disjoint sets
- • Graph algorithms: BFS, DFS, MSTs, shortest paths
The CS 225 study guide
How to study for UIUC CS 225, step by step.
- 1
Sharpen your C++ before the semester starts
CS 225 teaches the data structures from scratch but assumes the C++: pointers, dynamic memory, classes, ideally templates. Reviewing CS 128 material the month before is the highest-value prep available.
- 2
Start every MP the day it releases
The machine problems are substantial C++ projects with tight specs, and the autograder feedback loop rewards early starts disproportionately. Late starters fight the deadline and the debugger at once.
- 3
Run theory review continuously, not between MPs
CS 225's two tracks, MPs and CBTF theory exams, are designed to overlap. A standing 20-30 minutes of daily theory review keeps exam prep from ever competing head-on with an MP deadline.
- 4
Rebuild structures from logic, not memory
For AVL rotations, B-tree splits, and hashing strategies, practice reconstructing the operation from its invariant rather than memorizing steps. That's what survives the pressure of a proctored CBTF exam.
- 5
Use PrairieLearn practice before every CBTF window
The practice problems mirror the computer-based exam format closely. Work them until the question style is familiar, then book a CBTF slot early in the window while material is fresh.
Today
Today's CS 225 plan
What a Fennie Daily Plan looks like for CS 225. Yours is built from your own syllabus and adapts every day to your deadlines and progress.
First plan free, no card required. Fennie is independent and unaffiliated with your school.
FAQ
Is CS 225 the hardest class at UIUC?
It's the most commonly named candidate in the CS lower division. The combination of heavy C++ MPs and frequent theory exams demands sustained effort across the whole semester. It's also widely called the most valuable; its material dominates internship interviews.
How much time does CS 225 take per week?
Plan for 12-20 hours including lecture, labs, MP work, and exam prep, with spikes near MP deadlines. The students who suffer are those who start MPs late; the autograder feedback loop rewards early starts disproportionately.
How do I prepare for CS 225 before the semester?
Sharpen your C++: pointers, dynamic memory, classes, and ideally templates. Reviewing CS 128 material the month before is the highest-value preparation; the data structures are taught from scratch, but the C++ is assumed.
More UIUC courses
CS 124: Introduction to Computer Science I
CS 124 is UIUC's first programming course for CS majors, taught in Java or Kotlin (students choose), covering programming fundamentals through basic data structures and even Android development. It replaced the old CS 125 and is known for its polished homework infrastructure with daily small assignments.
CS 128: Introduction to Computer Science II
CS 128 is the second course in UIUC's CS sequence, moving students into C++ with memory management, object-oriented design, and an introduction to data structures and software engineering practice. It bridges CS 124 and the heavyweight CS 225.
CS 173: Discrete Structures
CS 173 is UIUC's discrete math course, covering logic, proofs, induction, sets, functions, graphs, and counting. It's the theory foundation for CS 225 and the algorithms coursework beyond. For most students it's their first proof-based mathematics.
CS 233: Computer Architecture
CS 233 is UIUC's computer architecture course, covering digital logic, MIPS assembly, processor datapaths, pipelining, and caches, taken after CS 225 in the CS core. It's known for its tightly engineered PrairieLearn workflow: practice problems with unlimited retries feeding frequent proctored quizzes.