Double Ended Queue Deque in DSA Python - Execution Trace

Concept Flow - Double Ended Queue Deque

Start

↓

Check Operation Type

↓

Add Front

↓

Update Front

↓

Check Remove?

↓

Remove Front

↓

Update Pointers

↓

Done

The deque allows adding or removing elements from both front and rear ends by updating pointers accordingly.

Execution Sample

DSA Python

from collections import deque
d = deque()
d.append(10)  # add rear
d.appendleft(5) # add front
d.pop()        # remove rear
d.popleft()    # remove front

This code shows adding and removing elements from both ends of a deque.

Execution Table

Step	Operation	Nodes in Deque	Pointer Changes	Visual State
1	Initialize empty deque	[]	head=None, tail=None	null
2	Add 10 at rear	[10]	head=10, tail=10	10 -> null
3	Add 5 at front	[5, 10]	head=5, tail=10	5 -> 10 -> null
4	Remove rear	[5]	head=5, tail=5	5 -> null
5	Remove front	[]	head=None, tail=None	null
6	Attempt remove front on empty	[]	Raises IndexError	null

💡 Deque is empty, no more elements to remove.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
deque	[]	[10]	[5, 10]	[5]	[]	[]
head	None	10	5	5	None	None
tail	None	10	10	5	None	None

Key Moments - 3 Insights

Why does the head pointer change when adding at the front but tail stays the same?

Because adding at the front inserts a new node before the current head, so head moves to new node, but tail remains at the last node (see Step 3 in execution_table).

What happens to pointers when the last element is removed?

Both head and tail become None, indicating the deque is empty (see Step 5 in execution_table).

Why can't we remove from front when deque is empty?

Because there are no nodes to remove, the operation raises an IndexError (see Step 6 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table at Step 3, what is the visual state of the deque?

A 10 -> 5 -> null

B 5 -> 10 -> null

C null

D 5 -> null

💡 Hint

Check the Visual State column at Step 3 in execution_table.

At which step does the deque become empty after removals?

A Step 4

B Step 5

C Step 6

D Step 3

💡 Hint

Look at Nodes in Deque column to find when it becomes [].

If we add 20 at rear after Step 5, what will be the new tail?

A 5

B None

C 20

D 10

💡 Hint

Adding at rear updates tail to new node (see Step 2 for similar operation).

Concept Snapshot

Double Ended Queue (Deque):
- Supports adding/removing from front and rear.
- Head points to front, tail points to rear.
- Adding front updates head; adding rear updates tail.
- Removing last element sets head and tail to None.
- Empty deque has no nodes and pointers are None.

Full Transcript

A double ended queue or deque lets you add or remove items from both ends. We start with an empty deque where head and tail pointers are None. When we add an element at the rear, it becomes both head and tail. Adding at the front inserts before head and updates head pointer. Removing from rear or front updates pointers accordingly. When the last element is removed, both pointers become None, showing the deque is empty. Trying to remove from an empty deque raises an IndexError. This step-by-step trace shows how the deque changes visually and how pointers move with each operation.

Practice

1. What is a deque in Python's collections module?

easy

A. A list that automatically sorts elements when inserted

B. A queue that only allows insertion at the rear and removal from the front

C. A stack that only allows insertion and removal from the top

D. A double-ended queue allowing insertion and removal from both ends

Solution

1. Step 1: Understand the deque concept

   A deque is a data structure that supports adding and removing elements from both the front and the back efficiently.

2. Step 2: Compare with other data structures

   Unlike a queue or stack, deque allows operations on both ends, not just one end.

3. Final Answer:

   A double-ended queue allowing insertion and removal from both ends -> Option D

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Remember: deque means double-ended queue [OK]

Common Mistakes:

• Confusing deque with stack or queue
• Thinking deque only works like a queue
• Assuming deque sorts elements automatically

2. Which of the following is the correct way to import and create an empty deque in Python?

easy

A. import collections\nd = collections.deque[]

B. from collections import deque\nd = deque()

C. from collections import Deque\nd = Deque()

D. import deque\nd = deque()

Solution

1. Step 1: Check the correct import statement

   The deque class is in the collections module and must be imported as from collections import deque.

2. Step 2: Verify deque creation syntax

   Creating an empty deque is done by calling deque() with parentheses, not brackets.

3. Final Answer:

   from collections import deque\nd = deque() -> Option B

4. Quick Check:

   Correct import and empty deque creation [OK]

Hint: Use 'from collections import deque' and call deque() [OK]

Common Mistakes:

• Using uppercase 'Deque' instead of 'deque'
• Trying to import deque directly without collections
• Using square brackets [] instead of parentheses ()

3. What will be the output of the following code?

from collections import deque
d = deque([1, 2, 3])
d.appendleft(0)
d.append(4)
print(list(d))

medium

A. [4, 1, 2, 3, 0]

B. [1, 2, 3, 0, 4]

C. [0, 1, 2, 3, 4]

D. [1, 2, 3, 4, 0]

Solution

1. Step 1: Trace deque initialization and appendleft

   Deque starts as [1, 2, 3]. After appendleft(0), it becomes [0, 1, 2, 3].

2. Step 2: Trace append operation

   After append(4), 4 is added to the right end, making deque [0, 1, 2, 3, 4].

3. Final Answer:

   [0, 1, 2, 3, 4] -> Option C

4. Quick Check:

   appendleft adds front, append adds end [OK]

Hint: appendleft adds front, append adds end [OK]

Common Mistakes:

• Confusing appendleft with append
• Assuming append adds to front
• Not converting deque to list before printing

4. Find the error in the following code snippet:

from collections import deque
d = deque()
d.appendleft(1)
d.append(2)
d.pop()
d.pop()
d.pop()

medium

A. Calling pop() on an empty deque causes an IndexError

B. appendleft() is not a valid method for deque

C. deque() cannot be initialized empty

D. append() requires two arguments

Solution

1. Step 1: Analyze deque operations

   Deque starts empty, then 1 is added to front, 2 to end. After two pops, deque is empty.

2. Step 2: Identify error on third pop

   The third pop tries to remove from empty deque, causing IndexError.

3. Final Answer:

   Calling pop() on an empty deque causes an IndexError -> Option A

4. Quick Check:

   pop on empty deque throws error [OK]

Hint: pop() on empty deque causes error [OK]

Common Mistakes:

• Thinking appendleft() is invalid
• Assuming deque can't be empty
• Believing append() needs two arguments

5. You have a deque initialized as d = deque([10, 20, 30, 40]). You want to rotate it so that the element 30 becomes the first element. Which code snippet achieves this correctly?

hard

A. d.rotate(-2)

B. d.rotate(2)

C. d.rotate(-3)

D. d.rotate(3)

Solution

1. Step 1: Understand deque rotation

   Positive rotate moves elements to the right; negative rotate moves to the left.

2. Step 2: Calculate rotation to bring 30 to front

   30 is at index 2. Rotating left by 2 (rotate(-2)) moves 30 to front.

3. Final Answer:

   d.rotate(-2) -> Option A

4. Quick Check:

   Negative rotate shifts left, bringing 30 front [OK]

Hint: Negative rotate shifts left; positive shifts right [OK]

Common Mistakes:

• Using positive rotate instead of negative
• Confusing rotation direction
• Not counting index positions correctly