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Project: Overview

Plan a mini app managing tasks with a stack and queue.

Project: Overview

In this project we build an undo system for a text editor. Stack, queue and state printing will accumulate.

Step 1: prepare empty stack. Step 2: add action. Step 3: undo. Step 4: print state.

Each lesson includes the previous solution; the code grows step by step.

Stack logic

A stack works with LIFO (last-in first-out) logic: the last added is the first removed. Ideal for an undo system.

Each action is pushed onto the stack. When undoing it is popped. The most recent action is undone first.

In Python a list is used: append for push, pop for pop.

Practice Empty stack

Write init_undo(). Return an empty list. Print [] with print(init_undo()).

Starter code

solution.py
def init_undo():
    # Empty undo stack

Sample solution

solution.py
def init_undo():
    # Empty undo stack
    return []
print(init_undo())

Expected output

[]

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Project: Overview: match the terms

Term Meaning
Step 1 Empty stack
Step 2 Push
Step 3 Undo
Step 4 Print

2 Which data structure does the undo system use?

  • Queue
  • Stack Correct answer
  • Tree
  • Hash table

Explanation: Stack

3 Push operation

Question 
stack.____(action)
Answer 
stack.append(action)

Explanation: append is right.

All lessons in Intermediate

1 Merge sort

  1. Divide and conquer
  2. Merge operation
  3. Full merge sort
  4. Summary - Merge sort

2 Quick sort

  1. Pivot selection
  2. Partitioning
  3. Recursive quick sort
  4. Summary - Quick sort

3 Heap sort

  1. What is a heap?
  2. Heapify
  3. Build heap and sort
  4. Summary - Heap sort

4 Counting sort

  1. Counting idea
  2. Count array
  3. Output array
  4. Summary - Counting sort

5 Radix sort

  1. Radix idea
  2. Digit pass
  3. Stable sub-sort
  4. Summary - Radix sort

6 Sorting project

  1. Project: Overview
  2. Score array
  3. Apply merge sort
  4. Print statistics

7 Tree

  1. What is a tree?
  2. Node and edge
  3. Binary tree
  4. Summary - Trees

8 Binary search tree

  1. BST rule
  2. BST insert
  3. BST search
  4. Summary - BST

9 AVL tree

  1. Balanced tree
  2. Rotation
  3. Balance after insert
  4. Summary - AVL tree

10 Heap

  1. Min and max heap
  2. Insert
  3. Extract
  4. Summary - Heap

11 Priority queue

  1. What is a priority queue?
  2. Heap-based implementation
  3. Push and pop
  4. Summary - Priority queue

12 Data structures project

  1. Project: Overview
  2. Undo stack
  3. Pending queue
  4. Print the state

13 Graph basics

  1. What is a graph?
  2. Adjacency list
  3. Adjacency matrix
  4. Summary - Graph

14 Breadth-first search

  1. BFS idea
  2. Using a queue
  3. Marking visited
  4. Summary - BFS

15 Depth-first search

  1. DFS idea
  2. Stack or recursion
  3. Backtracking
  4. Summary - DFS

16 Greedy algorithms

  1. Greedy approach
  2. Coin change
  3. Greedy limits
  4. Summary - Greedy

17 Divide and conquer

  1. Three steps
  2. Recursive structure
  3. Master theorem
  4. Summary - Divide and conquer

18 Backtracking

  1. What is backtracking?
  2. State tree
  3. Pruning
  4. Summary - Backtracking

19 Graph project

  1. Project: Overview
  2. City graph
  3. BFS for reachability
  4. Print the result

20 Intermediate final

  1. Challenge 1
  2. Challenge 2
  3. Challenge 3
  4. Challenge 4