Double Ended Queue Deque in DSA Typescript - Build from Scratch

Double Ended Queue (Deque) Operations

📖 Scenario: Imagine you are managing a line of customers at a coffee shop. Sometimes customers join the line at the front (VIPs), and sometimes at the end (regular customers). You also serve customers from the front or sometimes remove customers from the end if they leave early. This line is like a Double Ended Queue, called a Deque.

🎯 Goal: You will create a Deque using a TypeScript array, add customers to the front and back, remove customers from both ends, and finally display the current line of customers.

📋 What You'll Learn

Create a Deque as an array with initial customers

Add a customer to the front of the Deque

Add a customer to the back of the Deque

Remove a customer from the front of the Deque

Remove a customer from the back of the Deque

Print the final state of the Deque showing all customers in order

💡 Why This Matters

🌍 Real World

Double Ended Queues are used in real life for managing lines where people can join or leave from both ends, like VIP lines or task scheduling.

💼 Career

Understanding Deques helps in software development roles involving task scheduling, undo-redo features, and real-time data processing.

1

Create the initial Deque

Create a variable called deque as an array with these exact string elements in order: 'Alice', 'Bob', 'Charlie'

DSA Typescript

# Create the deque array with initial customers
# Your code here

Hint

Use square brackets [] to create an array and include the names as strings separated by commas.

2

Add customers to front and back

Add 'Diana' to the front of deque using unshift and add 'Evan' to the back of deque using push

DSA Typescript

const deque: string[] = ['Alice', 'Bob', 'Charlie'];
# Add 'Diana' to the front and 'Evan' to the back of deque
# Your code here

Hint

Use unshift to add at the front and push to add at the end of the array.

3

Remove customers from front and back

Remove one customer from the front of deque using shift and remove one customer from the back of deque using pop

DSA Typescript

const deque: string[] = ['Alice', 'Bob', 'Charlie'];
deque.unshift('Diana');
deque.push('Evan');
# Remove one customer from the front and one from the back
# Your code here

Hint

Use shift to remove from the front and pop to remove from the end of the array.

4

Print the final Deque state

Print the deque array to show the current order of customers using console.log(deque)

DSA Typescript

const deque: string[] = ['Alice', 'Bob', 'Charlie'];
deque.unshift('Diana');
deque.push('Evan');
deque.shift();
deque.pop();
# Print the deque array
# Your code here

Hint

Use console.log(deque) to print the array showing the current customers in order.

Practice

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

easy

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

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

C. A stack where elements can only be added or removed from the top

D. A list that allows random access to elements

Solution

1. Step 1: Understand the definition of a deque

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

2. Step 2: Compare with other data structures

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

3. Final Answer:

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

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 queue
• Thinking deque allows random access
• Assuming insertion only at one end

2. Which of the following is the correct TypeScript method to add an element at the front of a deque implemented as an array named deque?

easy

A. deque.shift(element);

B. deque.push(element);

C. deque.pop(element);

D. deque.unshift(element);

Solution

1. Step 1: Recall array methods for front insertion

   In TypeScript/JavaScript, unshift() adds an element at the start of an array.

2. Step 2: Verify other methods

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

3. Final Answer:

   deque.unshift(element); -> Option D

4. Quick Check:

   unshift() adds front [OK]

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

Common Mistakes:

• Using push() instead of unshift() for front
• Confusing pop() and shift() as insertion
• Passing arguments to pop() or shift()

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

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

medium

A. [20, 10, 30]

B. [20, 30]

C. [10, 30]

D. [30, 20]

Solution

1. Step 1: Trace each operation on deque

   Start: []
   push(10) -> [10]
   unshift(20) -> [20, 10]
   push(30) -> [20, 10, 30]
   shift() removes front -> removes 20 -> [10, 30]

2. Step 2: Final state of deque

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

3. Final Answer:

   [10, 30] -> Option C

4. Quick Check:

   After shift(), front removed -> [10, 30] [OK]

Hint: Remember shift() removes front element [OK]

Common Mistakes:

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

4. Identify the error in the following TypeScript code that implements a deque using an array:

const deque: number[] = [];
deque.push(5);
deque.unshift(10);
deque.pop(5);
console.log(deque);

medium

A. push() cannot be used on arrays

B. pop() should not have any arguments

C. unshift() removes elements, not adds

D. console.log syntax is incorrect

Solution

1. Step 1: Check usage of pop() method

   pop() removes the last element and does not take any arguments. Passing 5 is incorrect.

2. Step 2: Verify other methods and syntax

   push() adds element at end, unshift() adds at front, console.log syntax is correct.

3. Final Answer:

   pop() should not have any arguments -> Option B

4. Quick Check:

   pop() takes no arguments [OK]

Hint: pop() never takes arguments [OK]

Common Mistakes:

• Passing argument to pop()
• Confusing unshift() as removal
• Thinking push() is invalid on arrays

5. You want to implement a deque that supports adding and removing elements from both ends efficiently. Which data structure is best suited for this in TypeScript, and why?

hard

A. Use a linked list because it allows O(1) insertion and deletion at both ends

B. Use an array because it provides O(1) insertion and deletion at both ends

C. Use a binary tree because it supports fast search and insertion

D. Use a stack because it supports adding/removing from both ends

Solution

1. Step 1: Analyze efficiency of array for deque operations

   Arrays have O(1) insertion/deletion at the end but O(n) at the front due to shifting elements.

2. Step 2: Consider linked list advantages

   Linked lists allow O(1) insertion and deletion at both front and rear by adjusting pointers.

3. Step 3: Evaluate other options

   Binary trees are for search, not efficient deque operations; stacks only allow one end operations.

4. Final Answer:

   Use a linked list because it allows O(1) insertion and deletion at both ends -> Option A

5. Quick Check:

   Linked list = efficient deque [OK]

Hint: Linked list supports fast front and rear operations [OK]

Common Mistakes:

• Assuming arrays are efficient at front insertion
• Confusing stack with deque
• Choosing binary tree for deque operations