Double Ended Queue Deque in DSA Java - 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 ends.

beginner

Name two main operations you can perform on a Deque.

You can add elements (enqueue) and remove elements (dequeue) from both the front and the back ends.

beginner

How does a Deque differ from a regular Queue?

A regular Queue allows adding at the back and removing from the front only, while a Deque allows adding and removing from both ends.

beginner

What real-life example can help understand a Deque?

Think of a line of people where you can join or leave from the front or the back, like a double-ended waiting line.

beginner

In Java, which interface represents a Deque?

The java.util.Deque interface represents a double ended queue in Java.

Which operation is NOT possible in a Deque?

A Add element only at back

B Remove element from back

C Add element at front

D Remove element from front

In a Deque, you can add elements at both front and back, so adding only at back is not a limitation.

What is the output after these operations on an empty Deque: addFront(1), addBack(2), removeFront()?

A 1 -> 2 -> null

B 2 -> null

C 1 -> null

D null

After adding 1 at front and 2 at back, the Deque is 1 -> 2. Removing front removes 1, leaving 2.

Which Java class implements the Deque interface?

A LinkedList

B HashMap

C ArrayList

D Stack

LinkedList implements the Deque interface and supports double ended queue operations.

If you add elements 3, 5 at the back and 1 at the front, what is the order in the Deque?

A 1 -> 5 -> 3 -> null

B 3 -> 5 -> 1 -> null

C 5 -> 3 -> 1 -> null

D 1 -> 3 -> 5 -> null

Adding 1 at front puts it before 3 and 5, which were added at back.

What is the time complexity of adding or removing elements at either end of a Deque?

A O(n)

B O(log n)

C O(1)

D O(n^2)

Deque operations at both ends are done in constant time O(1).

Explain how a Deque works and give a simple real-life example.

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

Describe the difference between a Queue and a Deque.

Focus on where you can add and remove elements.

Practice

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

easy

A. A queue where elements can only be added at the rear

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

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 Deque

   A Deque allows insertion and deletion from both front and rear ends.

2. Step 2: Compare with other data structures

   Unlike queues or stacks, Deque supports operations at both ends.

3. Final Answer:

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

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 like arrays
• Assuming Deque only works from one end

2. Which of the following is the correct way to create a Deque in Java using the standard library?

easy

A. Deque<Integer> dq = new ArrayDeque<>();

B. Deque<Integer> dq = new Deque<>();

C. Deque<Integer> dq = new Stack<>();

D. Deque<Integer> dq = new Queue<>();

Solution

1. Step 1: Identify valid Deque implementations in Java

   ArrayDeque implements the Deque interface and is the preferred implementation.

2. Step 2: Check syntax correctness

   ArrayDeque is instantiated with new ArrayDeque<>(), which is correct syntax.

3. Final Answer:

   Deque<Integer> dq = new ArrayDeque<>(); -> Option A

4. Quick Check:

   ArrayDeque is standard Deque implementation [OK]

Hint: Use ArrayDeque for Deque in Java [OK]

Common Mistakes:

• Using Stack which is not a Deque
• Trying to instantiate interface Queue directly
• Trying to instantiate the Deque interface directly

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

Deque<String> dq = new ArrayDeque<>();
dq.addFirst("A");
dq.addLast("B");
dq.addFirst("C");
System.out.println(dq);

medium

A. [A, C, B]

B. [A, B, C]

C. [C, A, B]

D. [B, A, C]

Solution

1. Step 1: Trace addFirst and addLast operations

   addFirst("A") adds A at front: [A]
   addLast("B") adds B at rear: [A, B]
   addFirst("C") adds C at front: [C, A, B]

2. Step 2: Print the Deque

   Printing dq shows elements in order: [C, A, B]

3. Final Answer:

   [C, A, B] -> Option C

4. Quick Check:

   addFirst adds front, addLast adds rear [OK]

Hint: addFirst puts element at front, addLast at rear [OK]

Common Mistakes:

• Mixing order of addFirst and addLast
• Assuming addFirst adds at rear
• Confusing print output format

4. Identify the error in the following Java code using Deque:

Deque<Integer> dq = new ArrayDeque<>();
dq.addFirst(10);
dq.addLast(20);
dq.removeLast();
dq.removeFirst();
dq.removeFirst();

medium

A. No error, code runs fine

B. Compilation error due to wrong method names

C. Throws NullPointerException on removeLast()

D. Throws NoSuchElementException on last removeFirst()

Solution

1. Step 1: Trace the Deque operations

   Start: empty
   addFirst(10): [10]
   addLast(20): [10, 20]
   removeLast(): removes 20 -> [10]
   removeFirst(): removes 10 -> []
   removeFirst(): tries to remove from empty Deque

2. Step 2: Identify exception on removing from empty Deque

   removeFirst() on empty Deque throws NoSuchElementException

3. Final Answer:

   Throws NoSuchElementException on last removeFirst() -> Option D

4. Quick Check:

   Removing from empty Deque throws NoSuchElementException [OK]

Hint: Removing from empty Deque throws NoSuchElementException [OK]

Common Mistakes:

• Assuming removeFirst returns null instead of exception
• Confusing method names causing compile error
• Thinking removeLast throws NullPointerException

5. You want to implement a task scheduler that processes tasks in the order they arrive but sometimes needs to prioritize urgent tasks by adding them to the front. Which Deque operation combination is best suited for this scenario?

hard

A. Use addLast() for normal tasks and addFirst() for urgent tasks

B. Use addFirst() for normal tasks and addLast() for urgent tasks

C. Use only addLast() for all tasks

D. Use only addFirst() for all tasks

Solution

1. Step 1: Understand task order requirements

   Normal tasks should be processed in arrival order (queue behavior). Urgent tasks should be processed before normal tasks (front insertion).

2. Step 2: Match Deque operations to requirements

   addLast() adds normal tasks at rear (end), preserving arrival order.
   addFirst() adds urgent tasks at front, prioritizing them.

3. Final Answer:

   Use addLast() for normal tasks and addFirst() for urgent tasks -> Option A

4. Quick Check:

   Normal at rear, urgent at front [OK]

Hint: Normal tasks rear, urgent tasks front with addLast/addFirst [OK]

Common Mistakes:

• Reversing addFirst and addLast roles
• Using only one end for all tasks losing priority
• Confusing queue and stack behavior