Dequeue Operation in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Dequeue After Multiple Operations

What is the printed state of the dequeue after performing the following operations?

DSA Javascript

class Dequeue {
  constructor() {
    this.items = [];
  }
  addFront(element) {
    this.items.unshift(element);
  }
  addRear(element) {
    this.items.push(element);
  }
  removeFront() {
    return this.items.shift();
  }
  removeRear() {
    return this.items.pop();
  }
  print() {
    return this.items.join(' -> ') + ' -> null';
  }
}

const dq = new Dequeue();
dq.addRear(10);
dq.addRear(20);
dq.addFront(5);
dq.removeRear();
dq.addFront(3);
console.log(dq.print());

A 3 -> 5 -> 10 -> null

B 5 -> 3 -> 10 -> null

C 3 -> 5 -> 20 -> null

D 5 -> 10 -> null

💡 Hint

Trace each operation step-by-step, updating the dequeue contents.

Initially, the dequeue is empty.

addRear(10) adds 10 at rear: [10]

addRear(20) adds 20 at rear: [10, 20]

addFront(5) adds 5 at front: [5, 10, 20]

removeRear() removes 20 from rear: [5, 10]

addFront(3) adds 3 at front: [3, 5, 10]

Final print shows: 3 -> 5 -> 10 -> null

❓ Predict Output

intermediate

Result of Removing Elements from Dequeue

What will be the output after these dequeue operations?

DSA Javascript

class Dequeue {
  constructor() {
    this.items = [];
  }
  addFront(element) {
    this.items.unshift(element);
  }
  addRear(element) {
    this.items.push(element);
  }
  removeFront() {
    return this.items.shift();
  }
  removeRear() {
    return this.items.pop();
  }
  print() {
    return this.items.join(' -> ') + ' -> null';
  }
}

const dq = new Dequeue();
dq.addRear(1);
dq.addRear(2);
dq.addRear(3);
dq.removeFront();
dq.removeRear();
console.log(dq.print());

A 1 -> 2 -> 3 -> null

B 2 -> null

C 1 -> null

D 3 -> null

💡 Hint

Remember removeFront removes from the start, removeRear from the end.

Start with empty dequeue.

addRear(1), addRear(2), addRear(3) results in [1, 2, 3]

removeFront() removes 1: [2, 3]

removeRear() removes 3: [2]

Print shows: 2 -> null

🧠 Conceptual

advanced

Understanding Dequeue Underflow

What happens if you try to remove an element from the front of an empty dequeue?

A Throws a syntax error

B Adds an element to the rear instead

C Returns undefined or null depending on implementation

D Removes the last element from the rear

💡 Hint

Think about what happens when you remove from an empty list or array.

Removing from an empty dequeue usually returns undefined or null because there is nothing to remove.

It does not throw syntax errors or add elements.

❓ Predict Output

advanced

Output After Complex Dequeue Operations

What is the final printed state of the dequeue after these operations?

DSA Javascript

class Dequeue {
  constructor() {
    this.items = [];
  }
  addFront(element) {
    this.items.unshift(element);
  }
  addRear(element) {
    this.items.push(element);
  }
  removeFront() {
    return this.items.shift();
  }
  removeRear() {
    return this.items.pop();
  }
  print() {
    return this.items.join(' -> ') + ' -> null';
  }
}

const dq = new Dequeue();
dq.addRear(7);
dq.addFront(3);
dq.addRear(9);
dq.removeFront();
dq.addFront(1);
dq.removeRear();
console.log(dq.print());

A 3 -> 7 -> 9 -> null

B 7 -> 9 -> null

C 1 -> 3 -> 7 -> null

D 1 -> 7 -> null

💡 Hint

Track each operation carefully, updating the dequeue contents.

Start empty.

addRear(7): [7]

addFront(3): [3, 7]

addRear(9): [3, 7, 9]

removeFront(): removes 3 → [7, 9]

addFront(1): [1, 7, 9]

removeRear(): removes 9 → [1, 7]

Print: 1 -> 7 -> null

🔧 Debug

expert

Identify the Error in Dequeue Implementation

What error will this code produce when trying to remove from an empty dequeue?

DSA Javascript

class Dequeue {
  constructor() {
    this.items = [];
  }
  removeFront() {
    return this.items.shift();
  }
}

const dq = new Dequeue();
console.log(dq.removeFront().toString());

A TypeError: Cannot read property 'toString' of undefined

B SyntaxError: Unexpected token

C No error, prints 'undefined'

D ReferenceError: dq is not defined

💡 Hint

What does shift() return when the array is empty? What happens when you call toString() on that?

When removing from empty array, shift() returns undefined.

Calling toString() on undefined causes a TypeError.

Practice

1. What is a dequeue in data structures?

easy

A. A stack that allows only last-in, first-out operations

B. A queue where elements can only be added at the front

C. A list that only allows insertion at the middle

D. A double-ended queue where elements can be added or removed from both ends

Solution

1. Step 1: Understand the definition of dequeue

   A dequeue is a data structure that allows insertion and deletion from both front and rear ends.

2. Step 2: Compare options with definition

   A double-ended queue where elements can be added or removed from both ends correctly describes a dequeue. Options A, B, and D describe other structures or incorrect behaviors.

3. Final Answer:

   A double-ended queue where elements can be added or removed from both ends -> Option D

4. Quick Check:

   Dequeue = Double-ended queue [OK]

Hint: Dequeue means double-ended queue, both ends allowed [OK]

Common Mistakes:

• Confusing dequeue with stack or normal queue
• Thinking elements can only be added at one end
• Assuming dequeue is a type of list

2. Which of the following is the correct syntax to add an element 5 at the front of a dequeue named dq in JavaScript using an array?

easy

A. dq.pushFront(5);

B. dq.unshift(5);

C. dq.addFront(5);

D. dq.insertFront(5);

Solution

1. Step 1: Recall JavaScript array methods for front insertion

   JavaScript arrays use unshift() to add elements at the front.

2. Step 2: Match method with options

   Only dq.unshift(5); uses unshift(5), which is correct. Other options are invalid methods.

3. Final Answer:

   dq.unshift(5); -> Option B

4. Quick Check:

   Array front insert = unshift() [OK]

Hint: Use unshift() to add at front in JS arrays [OK]

Common Mistakes:

• Using pushFront which is not a JS array method
• Confusing push (adds at end) with front insertion
• Assuming addFront or insertFront exist in JS arrays

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

let 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 dequeue array

   Start with empty dq = []. push(10) adds 10 at end: [10]. unshift(20) adds 20 at front: [20, 10]. push(30) adds 30 at end: [20, 10, 30]. shift() removes front element (20): [10, 30].

2. Step 2: Determine final state of dq

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

3. Final Answer:

   [10, 30] -> Option A

4. Quick Check:

   After shift(), front removed, dq = [10, 30] [OK]

Hint: Remember shift() removes front, push() adds end [OK]

Common Mistakes:

• Confusing shift() with pop()
• Mixing order of unshift and push
• Forgetting shift() removes front element

4. Identify the error in the following dequeue operation code:

let dq = [];
dq.push(1);
dq.popFront();
console.log(dq);

medium

A. popFront() is not a valid JavaScript array method

B. push() cannot be used on empty arrays

C. popFront() should be pop() to remove last element

D. dq should be initialized as a Dequeue object, not array

Solution

1. Step 1: Check JavaScript array methods used

   JavaScript arrays do not have a method named popFront(). The correct method to remove front element is shift().

2. Step 2: Identify the error

   Using popFront() causes a runtime error as it is undefined.

3. Final Answer:

   popFront() is not a valid JavaScript array method -> Option A

4. Quick Check:

   popFront() invalid in JS arrays [OK]

Hint: Use shift() to remove front, pop() for end [OK]

Common Mistakes:

• Assuming popFront() exists like pop()
• Confusing pop() with front removal
• Not knowing shift() removes front element

5. You have a dequeue implemented as a JavaScript array dq with elements [5, 10, 15, 20]. You want to remove the element 5 from the front and add 25 at the rear, then remove the element 20 from the rear and add 30 at the front. What will be the final state of dq?

hard

A. [30, 5, 10, 15, 25]

B. [10, 15, 25, 30]

C. [30, 10, 15, 25]

D. [10, 15, 20, 25, 30]

Solution

1. Step 1: Remove front element

   Start dq = [5, 10, 15, 20]. Remove front element 5 using shift(): dq = [10, 15, 20]. Rear is now 20.

2. Step 2: Remove rear, add rear, add front

   Remove rear element 20 using pop(): dq = [10, 15]. Add 25 at rear using push(): dq = [10, 15, 25]. Add 30 at front using unshift(): dq = [30, 10, 15, 25].

3. Final Answer:

   [30, 10, 15, 25] -> Option C

4. Quick Check:

   Front removed 5, rear removed 20, added 30 front, 25 rear [OK]

Hint: Use shift/pop for removal, unshift/push for addition [OK]

Common Mistakes:

• Removing wrong elements from front or rear
• Mixing order of operations
• Using pop() instead of shift() for front removal