CodeBlanks Logo Icon CodeBlanks
Login Register

What is a priority queue?

A queue where each element has a priority.

What is a priority queue?

A priority queue is a data structure where each element has a priority and the highest (or lowest) priority element comes out first.

Unlike a regular queue (FIFO), the dequeue order depends on priority. Implemented with a heap, insert and extract are O(log n).

Used in task schedulers, Dijkstra's algorithm and Huffman coding.

Heap-based implementation

A priority queue is most efficiently implemented with a heap. A min-heap keeps the lowest priority at the root; a max-heap the highest.

Insert: add to heap + sift-up, O(log n). Extract: take root + move last to root + sift-down, O(log n).

Peek (look at root) is O(1). This balances against a sorted array.

Practice Peek

Write peek(arr). Return the root (smallest). Print 1 with print(peek([1, 3, 2, 5, 4])).

Starter code

solution.py
def peek(arr):
    # Return root

Sample solution

solution.py
def peek(arr):
    # Return root
    return arr[0]
print(peek([1, 3, 2, 5, 4]))

Expected output

1

Questions and answers

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

1 What does a priority queue dequeue first?

  • The first inserted
  • The highest priority Correct answer
  • The last inserted
  • Random

Explanation: The highest priority

2 Peek

Question 
return arr[____]
Answer 
return arr[0]

Explanation: 0 is right.

3 What is a priority queue: match the terms

Term Meaning
Push Insert + sift-up
Pop Take root + sift-down
Peek Read root
Heap Foundation

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