Double Ended Queue Deque in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Double Ended Queue (Deque)?

A Double Ended Queue, or Deque, is a data structure where elements can be added or removed from both the front and the back, like a line where people can enter or leave from either end.

beginner

Name two main operations you can perform on a Deque.

You can add elements to the front or back (called enqueue front and enqueue back) and remove elements from the front or back (called dequeue front and dequeue back).

beginner

How does a Deque differ from a regular Queue?

A regular Queue only allows adding at the back and removing from the front (like a line at a store). A Deque allows adding and removing from both ends.

beginner

What real-life example can help you understand a Deque?

Think of a train with doors at both ends. Passengers can enter or exit from the front or back doors, just like adding or removing elements from either end of a Deque.

intermediate

In TypeScript, which data structure can be used to implement a Deque efficiently?

An array can be used with methods like push, pop, unshift, and shift to add or remove elements from both ends, implementing a Deque.

Which operation is NOT possible in a Deque?

A Add element at the front

B Remove element from the back

C Add element only at the back

D Remove element only from the middle

Deques allow adding/removing elements only at the front or back, not from the middle.

What method would you use to add an element to the front of a Deque implemented with an array in TypeScript?

A unshift()

B pop()

C push()

D shift()

unshift() adds an element to the front of an array.

If you remove an element from the back of a Deque, which array method is used in TypeScript?

A shift()

B push()

C pop()

D unshift()

pop() removes the last element from an array.

Which of these is a correct description of a Deque?

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

B A structure that only stores unique elements

C A structure where elements can only be removed from the front

D A structure where elements can only be added at the back

A Deque allows adding and removing elements from both front and back.

Which real-life example best illustrates a Deque?

A A single line at a ticket counter

B A train with doors at both ends where passengers enter and exit

C A stack of plates where you add and remove from the top

D A bookshelf where books are only added at one side

A train with doors at both ends is like a Deque where you can add or remove from both ends.

Explain what a Double Ended Queue (Deque) is and how it works.

Think about a line where people can enter or leave from both front and back.

Describe how you would implement a Deque using a TypeScript array and which methods you would use for each operation.

Remember which array methods add or remove elements from front or back.

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