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Queue

Introduction​

A queue is a linear data structure that follows the First-In, First-Out (FIFO) principle. The first element added is the first one to be removed.

Key Operations​

  • Enqueue: Add an element to the end of the queue (O(1))
  • Dequeue: Remove the element from the front of the queue (O(1))
  • Peek/Front: View the front element without removing it (O(1))
  • IsEmpty: Check if the queue is empty

Example (Python)​

from collections import deque

queue = deque()

# Enqueue elements
queue.append(10)
queue.append(20)
queue.append(30)

# Peek at the front
print(queue[0])  # Output: 10

# Dequeue elements
print(queue.popleft())  # Output: 10
print(queue.popleft())  # Output: 20

# Check if empty
if not queue:
    print("Queue is empty!")

Applications​

  • Task scheduling (print queue, CPU scheduling)
  • Breadth-first search (BFS) in graphs
  • Handling requests in web servers
  • Buffer management

When to Use a Queue​

  • When you need to process items in the order they arrive
  • When implementing algorithms like BFS

Limitations​

  • Only the front and rear elements are accessible
  • Not suitable for random access
  • Stack: Follows Last-In, First-Out (LIFO)
  • Deque: Double-ended queue, allows insertion/removal from both ends