Double Ended Queue Deque in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

deque.[1](10);

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

Apop

Bpush

Cunshift

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of unshift adds to the back, not front.

Using pop or shift removes elements instead of adding.

The unshift method adds an element to the front of an array, which works as the front of the deque.

2fill in blank

medium

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

DSA Javascript

let removed = deque.[1]();

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

Ashift

Bpop

Cunshift

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift removes from the front, not the back.

Using push or unshift adds elements instead of removing.

The pop method removes the last element from an array, which corresponds to the back of the deque.

3fill in blank

hard

Fix the error in the code to correctly add an element to the back of the deque.

DSA Javascript

deque.[1](20);

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

Apush

Bpop

Cunshift

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop or shift which remove elements instead of adding.

Using unshift adds to the front, not the back.

The push method adds an element to the end of the array, which is the back of the deque.

4fill in blank

hard

Fill both blanks to create a function that removes from front and adds to back of the deque.

DSA Javascript

function rotateDeque(deque) {
  let front = deque.[1]();
  deque.[2](front);
}

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

Ashift

Bpop

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of shift removes from back, not front.

Using unshift adds to front, not back.

The shift method removes the front element, and push adds it to the back, rotating the deque.

5fill in blank

hard

Fill all three blanks to create a function that adds to front, removes from back, and returns the removed element.

DSA Javascript

function addFrontRemoveBack(deque, value) {
  deque.[1](value);
  let removed = deque.[2]();
  return removed;
}

let deque = [1, 2, 3];
let result = addFrontRemoveBack(deque, 0);
console.log(deque); // Output: [3]

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

Aunshift

Bpop

C[0, 1, 2]

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift instead of pop removes from front, not back.

Incorrect output array after operations.

The function adds value to the front using unshift, removes the last element with pop, and returns it. The final deque after adding 0 to front and removing 3 from back is [0, 1, 2].

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 can only be added at the rear and removed from the front.

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

D. A list that allows random access to elements by index.

Solution

1. Step 1: Understand the definition of Deque

   A Deque allows insertion and deletion at both ends, unlike a queue or stack which restrict operations to one end.

2. Step 2: Compare with other data structures

   Queue allows operations only at front and rear separately; stack only at one end; list allows random access but not double-ended insertion/removal.

3. Final Answer:

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

4. Quick Check:

   Deque = double-ended queue [OK]

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

Common Mistakes:

• Confusing deque with stack or simple queue
• Thinking deque allows random access by index
• Assuming deque operations only at one end

2. Which of the following is the correct way to add an element to the front of a deque named dq in JavaScript using an array?

easy

A. dq.unshift(element);

B. dq.pop(element);

C. dq.push(element);

D. dq.shift(element);

Solution

1. Step 1: Identify array methods for front insertion

   In JavaScript, unshift() adds an element to the start (front) of an array.

2. Step 2: Check other methods

   push() adds to the end, pop() removes from end, shift() removes from front.

3. Final Answer:

   dq.unshift(element); -> Option A

4. Quick Check:

   unshift adds front element [OK]

Hint: Use unshift() to add front element in JS array [OK]

Common Mistakes:

• Using push() instead of unshift() for front insertion
• Confusing pop() and shift() as insertion methods
• Passing argument to pop() or shift() which don't take any

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

const dq = [];
dq.push(10);
dq.unshift(20);
dq.push(30);
dq.shift();
console.log(dq);

medium

A. [10, 30]

B. [20, 10, 30]

C. [20, 30]

D. [10, 20, 30]

Solution

1. Step 1: Trace each operation on the deque

   Start with empty array [].
   push(10) => [10]
   unshift(20) => [20, 10]
   push(30) => [20, 10, 30]
   shift() removes front element 20 => [10, 30]

2. Step 2: Final state of dq

   After all operations, dq contains [10, 30].

3. Final Answer:

   [10, 30] -> Option A

4. Quick Check:

   After shift(), front 20 removed, left [10, 30] [OK]

Hint: Track each push/unshift/shift step carefully [OK]

Common Mistakes:

• Forgetting shift() removes front element
• Mixing order of push and unshift effects
• Assuming shift() removes last element

4. Identify the error in the following code snippet that tries to remove an element from the rear of a deque implemented as an array named dq:

dq.removeLast();

medium

A. dq.pop() should be used instead of removeLast().

B. removeLast() is not a valid JavaScript array method.

C. removeLast() requires an argument to specify the element.

D. dq should be a linked list to use removeLast().

Solution

1. Step 1: Check JavaScript array methods for removing last element

   JavaScript arrays do not have a method named removeLast(). The correct method to remove the last element is pop().

2. Step 2: Analyze other options

   Linked list is unrelated here; removeLast() does not take arguments; pop() is the correct method.

3. Final Answer:

   removeLast() is not a valid JavaScript array method. -> Option B

4. Quick Check:

   pop() removes last element, removeLast() invalid [OK]

Hint: Use pop() to remove last element in JS array [OK]

Common Mistakes:

• Assuming removeLast() exists in JS arrays
• Trying to pass arguments to pop() or removeLast()
• Confusing linked list methods with array methods

5. You want to implement a deque that supports the following operations efficiently: addFront, addRear, removeFront, removeRear. Which data structure is best suited for this and why?

hard

A. Array, because it allows fast insertion and deletion at both ends.

B. Stack, because it supports adding and removing elements efficiently.

C. Singly linked list, because it allows fast insertion and deletion at both ends.

D. Doubly linked list, because it allows fast insertion and deletion at both ends without shifting elements.

Solution

1. Step 1: Analyze operation requirements

   We need fast insertion and deletion at both front and rear ends.

2. Step 2: Compare data structures

   Array requires shifting elements for front operations, making it inefficient.
   Singly linked list allows fast insertion/removal only at one end efficiently.
   Doubly linked list allows fast insertion/removal at both ends without shifting.
   Stack supports operations only at one end.

3. Final Answer:

   Doubly linked list, because it allows fast insertion and deletion at both ends without shifting elements. -> Option D

4. Quick Check:

   Doubly linked list = best for deque operations [OK]

Hint: Use doubly linked list for efficient double-ended ops [OK]

Common Mistakes:

• Choosing array ignoring shifting cost
• Assuming singly linked list supports both ends efficiently
• Confusing stack with deque