Deque in Python

A deque stands for Double-Ended Queue. It is a special type of data structure that allows you to add and remove elements from both ends efficiently.

Unlike normal queues (which usually follow First In, First Out), a deque supports both FIFO and LIFO operations. This makes it very flexible and useful in real-world applications like task scheduling, sliding window problems and real-time data processing.

Example:

from collections import deque # Declaring deque  de = deque(['name','age','DOB']) print(de) 

deque(['name', 'age', 'DOB']) 

Why Do We Need deque

• It supports O(1) time for adding/removing elements from both ends.
• It is more efficient than lists for front-end operations.
• It can function as both a queue (FIFO) and a stack (LIFO).
• Ideal for scheduling, sliding window problems and real-time data processing.
• It offers powerful built-in methods like appendleft(), popleft() and rotate().

Types of Restricted Deque Input

• Input Restricted Deque:  Input is limited at one end while deletion is permitted at both ends.
• Output Restricted Deque: output is limited at one end but insertion is permitted at both ends.

Operations on deque 

Here’s a table listing built-in operations of a deque in Python with descriptions and their corresponding time complexities:

Appending and Deleting Dequeue Items

• append(x): Adds x to the right end of the deque.
• appendleft(x): Adds x to the left end of the deque.
• extend(iterable): Adds all elements from the iterable to the right end.
• extendleft(iterable): Adds all elements from the iterable to the left end (in reverse order).
• remove(value): Removes the first occurrence of the specified value from the deque. If value is not found, it raises a ValueError.
• pop(): Removes and returns an element from the right end.
• popleft(): Removes and returns an element from the left end.
• clear(): Removes all elements from the deque.

from collections import deque dq = deque([10, 20, 30]) # Add elements to the right dq.append(40) # Add elements to the left dq.appendleft(5) # extend(iterable) dq.extend([50, 60, 70]) print("After extend([50, 60, 70]):", dq) # extendleft(iterable) dq.extendleft([0, 5]) print("After extendleft([0, 5]):", dq) # remove method dq.remove(20) print("After remove(20):", dq) # Remove elements from the right dq.pop() # Remove elements from the left dq.popleft() print("After pop and popleft:", dq) # clear() - Removes all elements from the deque dq.clear() # deque: [] print("After clear():", dq) 

Output:

After extend([50, 60, 70]): deque([5, 10, 20, 30, 40, 50, 60, 70])
After extendleft([0, 5]): deque([5, 0, 5, 10, 20, 30, 40, 50, 60, 70])
After remove(20): deque([5, 0, 5, 10, 30, 40, 50, 60, 70])
After pop and popleft: deque([0, 5, 10, 30, 40, 50, 60])
After clear(): deque([])

Accessing Item and length of deque

• Indexing: Access elements by position using positive or negative indices.
• len(): Returns the number of elements in the deque.

import collections dq = collections.deque([1, 2, 3, 3, 4, 2, 4]) # Accessing elements by index print(dq[0]) print(dq[-1]) # Finding the length of the deque print(len(dq)) 

1 4 7 

Count, Rotation and Reversal of a deque

• count(value): This method counts the number of occurrences of a specific element in the deque.
• rotate(n): This method rotates the deque by n steps. Positive n rotates to the right and negative n rotates to the left.
• reverse(): This method reverses the order of elements in the deque.

from collections import deque # Create a deque dq = deque([10, 20, 30, 40, 50, 20, 30, 20]) # 1. Counting occurrences of a value print(dq.count(20)) # Occurrences of 20 print(dq.count(30)) # Occurrences of 30 # 2. Rotating the deque dq.rotate(2) # Rotate the deque 2 steps to the right print(dq) dq.rotate(-3) # Rotate the deque 3 steps to the left print(dq) # 3. Reversing the deque dq.reverse() # Reverse the deque print(dq) 

3 2 deque([30, 20, 10, 20, 30, 40, 50, 20]) deque([20, 30, 40, 50, 20, 30, 20, 10]) deque([10, 20, 30, 20, 50, 40, 30, 20])