---
title: "Lecture notes — 2026-04-07 (abstract data types)"
project: "cs101-intro"
tags:
  - lecture
  - study-notes
  - cs101
date: 2026-04-07
---

# Lecture 2026-04-07 — Abstract data types

**Source** — Live lecture + **import from audio transcription** (Otter.ai export, cleaned 2026-04-07 evening).

## Summary (5 bullets)

- ADT = **behavior contract** separate from implementation.
- **List ADT** can be array-backed or linked; tradeoffs: cache locality vs. insert cost.
- **Iterator** pattern hides internal structure from client code.
- Big-O today focused on **worst case** unless stated otherwise.
- Prof emphasized: **choose the ADT first**, then pick the implementation.

## Detailed notes

- **Array list** — doubling strategy keeps append amortized O(1); wasted space bounded by a constant factor.
- **Singly linked list** — O(1) insert at head; need tail pointer for O(1) enqueue in naive queue.
- **Invariant** example: `size` field must equal length walked from head.

## Questions for office hours

- Why not use Python `list` for everything in homework if industry does?  
  **Answer (from slide footer)** — Pedagogy: you implement to see *why* the optimizations exist.

## Homework link

Maps directly to `assignments/week-3-data-structures.md` — start `Stack` tonight.