Double Ended Queue Deque in DSA C++ - Execution Trace

Concept Flow - Double Ended Queue Deque

Start

↓

Check Operation Type

↓

Insert Front

↓

Update Front Pointer

↓

Add Element at Front

↓

Check Remove Operation

↓

Remove Front

↓

Update Front Pointer

↓

Return Removed Element

↓

End

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

Execution Sample

DSA C++

deque.push_back(10);
deque.push_front(20);
deque.push_back(30);
deque.pop_front();
deque.pop_back();

This code adds 10 at rear, 20 at front, 30 at rear, then removes from front and rear.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	push_back(10)	10	rear -> 10, front -> 10	10 -> null
2	push_front(20)	20 -> 10	front -> 20	20 -> 10 -> null
3	push_back(30)	20 -> 10 -> 30	rear -> 30	20 -> 10 -> 30 -> null
4	pop_front()	10 -> 30	front -> 10	10 -> 30 -> null
5	pop_back()	10	rear -> 10	10 -> null
6	End	10	No change	10 -> null

💡 No more operations, deque state stable with one element 10.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
front	null	10	20	20	10	10	10
rear	null	10	10	30	30	10	10
size	0	1	2	3	2	1	1

Key Moments - 3 Insights

Why does the front pointer change when we push_front but rear stays the same?

When pushing at front (see step 2 in execution_table), we add a new node before the current front, so front pointer updates to new node. Rear pointer remains unchanged because the end of the list is not affected.

Why after pop_front (step 4) does the front pointer move to the next node?

Pop_front removes the first node, so front pointer must move to the next node to reflect the new start of the deque, as shown in step 4.

Why does pop_back (step 5) update rear pointer but not front pointer?

Pop_back removes the last node, so rear pointer moves to the previous node. Front pointer stays the same because the start of the deque is unchanged.

Visual Quiz - 3 Questions

Look at the execution_table, what is the visual state after step 3?

A 20 -> 10 -> 30 -> null

B 10 -> 20 -> 30 -> null

C 30 -> 10 -> 20 -> null

D 10 -> 30 -> null

💡 Hint

Check the 'Visual State' column for step 3 in execution_table.

At which step does the size of the deque become 2 according to variable_tracker?

A After 1

B After 2

C After 3

D After 4

💡 Hint

Look at the 'size' row in variable_tracker and find when it changes to 2.

If we remove pop_front() operation, what would be the rear pointer after step 5?

A 30

B 10

C 20

D null

💡 Hint

Without pop_front(), after step 3 rear=30, but step 5 pop_back() updates rear to 10. Check variable_tracker 'rear' row (step 3: 30, after 5: 10).

Concept Snapshot

Double Ended Queue (Deque):
- Supports insertion/removal at both front and rear.
- push_front adds at front; push_back adds at rear.
- pop_front removes from front; pop_back removes from rear.
- Front and rear pointers update accordingly.
- Size tracks number of elements.

Full Transcript

A double ended queue, or deque, allows adding and removing elements from both ends. When we add at the front, the front pointer moves to the new node, and when we add at the rear, the rear pointer moves. Removing from front or rear updates pointers to keep track of the current ends. The size variable keeps count of elements. This trace shows step-by-step how nodes are added and removed, and how pointers and size change.

Practice

1. What is a Double Ended Queue (Deque)?

easy

A. A stack where elements are added and removed only from the top

B. A queue where elements are added only at the rear and removed from the front

C. A data structure where elements can be added or removed from both ends

D. A list where elements can be accessed randomly by index

Solution

1. Step 1: Understand the definition of Deque

   A Deque allows insertion and deletion at both front and rear ends, unlike a queue or stack.

2. Step 2: Compare with other data structures

   Queue allows operations at one end for insertion and the other for deletion; stack allows operations only at one end.

3. Final Answer:

   A data structure where elements can be added or removed from both ends -> Option C

4. Quick Check:

   Deque = double-ended queue allowing both ends operations [OK]

Hint: Deque means double-ended queue, both ends usable [OK]

Common Mistakes:

• Confusing deque with stack or queue
• Thinking deque allows random access like arrays
• Assuming deque only works like a queue

2. Which of the following is the correct syntax to declare a deque of integers in C++?

easy

A. deque<int> d = new deque();

B. deque<int> d;

C. deque<int> d[];

D. deque d<int>;

Solution

1. Step 1: Recall C++ STL deque declaration syntax

   The correct syntax uses template syntax: deque<int> d; declares a deque of integers named d.

2. Step 2: Check other options for syntax errors

   'deque<int> d = new deque();' uses invalid 'new' syntax for STL containers, 'deque d<int>;' has wrong template placement, 'deque<int> d[];' declares an array of deques incorrectly.

3. Final Answer:

   deque<int> d; -> Option B

4. Quick Check:

   Correct C++ deque declaration = deque<int> d; [OK]

Hint: Use template syntax: deque<Type> name; [OK]

Common Mistakes:

• Placing template brackets incorrectly
• Using Java or other language syntax
• Declaring arrays of deques without proper syntax

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

#include <iostream>
#include <deque>
using namespace std;
int main() {
  deque<int> d;
  d.push_back(10);
  d.push_front(20);
  d.push_back(30);
  d.pop_front();
  for (auto x : d) cout << x << " ";
  return 0;
}

medium

A. 10 30

B. 30 10

C. 20 10 30

D. 10 20 30

Solution

1. Step 1: Trace deque operations step-by-step

   Initially empty deque. push_back(10) -> [10]. push_front(20) -> [20, 10]. push_back(30) -> [20, 10, 30]. pop_front() removes 20 -> [10, 30].

2. Step 2: Print elements in deque

   Loop prints elements in order: 10 30

3. Final Answer:

   10 30 -> Option A

4. Quick Check:

   After pop_front, deque = 10 30 [OK]

Hint: Track push_front/back and pop_front carefully [OK]

Common Mistakes:

• Forgetting pop_front removes front element
• Mixing order of push_front and push_back
• Assuming pop_front removes from back

4. Identify the error in the following C++ code using deque:

#include <deque>
using namespace std;
int main() {
  deque<int> d;
  d.push_front(5);
  d.pop_back();
  d.pop_back();
  return 0;
}

medium

A. deque<int> d; is invalid declaration

B. push_front() is not a valid deque method

C. Missing #include <iostream> for cout

D. Calling pop_back() on empty deque causes undefined behavior

Solution

1. Step 1: Analyze deque operations

   After push_front(5), deque has one element [5]. First pop_back() removes 5, deque becomes empty []. Second pop_back() is called on empty deque.

2. Step 2: Understand behavior of pop_back on empty deque

   Calling pop_back() on empty deque causes undefined behavior or runtime error.

3. Final Answer:

   Calling pop_back() on empty deque causes undefined behavior -> Option D

4. Quick Check:

   pop_back on empty deque = error [OK]

Hint: Never pop from empty deque to avoid errors [OK]

Common Mistakes:

• Assuming pop_back on empty deque is safe
• Confusing push_front with invalid method
• Ignoring missing iostream is irrelevant here

5. You want to implement a sliding window maximum using a deque in C++. Which deque operation is essential to maintain the window elements in decreasing order?

hard

A. Remove elements from the back while they are smaller than the new element

B. Remove elements from the front while they are smaller than the new element

C. Add new elements only at the front

D. Add new elements only at the back without removal

Solution

1. Step 1: Understand sliding window maximum logic

   To keep maximum at front, remove all smaller elements from back before adding new element at back.

2. Step 2: Identify correct deque operation

   Removing smaller elements from back maintains decreasing order, ensuring front is max.

3. Final Answer:

   Remove elements from the back while they are smaller than the new element -> Option A

4. Quick Check:

   Sliding window max uses back removals for order [OK]

Hint: Remove smaller from back to keep max at front [OK]

Common Mistakes:

• Removing from front instead of back
• Adding elements without removing smaller ones
• Adding only at front breaks order