Double Ended Queue Deque in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element at the front of the deque.

DSA C++

std::deque<int> dq;
dq.[1](10);

Drag options to blanks, or click blank then click option'

Apop_back

Bpush_back

Cpop_front

Dpush_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using push_back instead of push_front adds element at the end, not front.

Using pop functions instead of push functions.

To add an element at the front of a deque, use push_front.

2fill in blank

medium

Complete the code to remove an element from the back of the deque.

DSA C++

std::deque<int> dq = {1, 2, 3};
dq.[1]();

Drag options to blanks, or click blank then click option'

Apop_back

Bpush_front

Cpop_front

Dpush_back

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_front removes from the front, not the back.

Using push functions instead of pop functions.

To remove an element from the back of a deque, use pop_back.

3fill in blank

hard

Fix the error in accessing the first element of the deque.

DSA C++

std::deque<int> dq = {5, 10, 15};
int first = dq.[1]();

Drag options to blanks, or click blank then click option'

Aback

Bpop_front

Cfront

Dpush_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using back() returns the last element, not the first.

Using pop_front() removes the element instead of accessing it.

To access the first element without removing it, use front().

4fill in blank

hard

Fill both blanks to insert 20 at the back and then remove from the front.

DSA C++

std::deque<int> dq;
dq.[1](20);
dq.[2]();

Drag options to blanks, or click blank then click option'

Apush_back

Bpop_back

Cpop_front

Dpush_front

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up push_front and push_back.

Using pop_back instead of pop_front for removal.

Use push_back to add at the back and pop_front to remove from the front.

5fill in blank

hard

Fill all three blanks to create a deque with 1, add 2 at front, then remove first element.

DSA C++

std::deque<int> dq = {1};
dq.[1](2);
dq.[2]();
int last = dq.[3]();

Drag options to blanks, or click blank then click option'

Apush_front

Bpop_back

Cback

Dpop_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_back instead of pop_front for removal.

Accessing front instead of back for last element.

Use push_front to add 2 at front, pop_front to remove first element, and back to access last element.

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