Dequeue Operation in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Dequeue after multiple insertions and deletions

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

DSA Java

import java.util.Deque;
import java.util.LinkedList;

public class Main {
    public static void main(String[] args) {
        Deque<Integer> dq = new LinkedList<>();
        dq.addFirst(10);
        dq.addLast(20);
        dq.addFirst(5);
        dq.removeLast();
        dq.addLast(15);
        System.out.println(dq);
    }
}

A [5, 10, 15]

B [10, 5, 15]

C [5, 10, 20]

D [10, 20, 15]

💡 Hint

Remember addFirst adds to front, addLast adds to end, removeLast removes from end.

Initially, dq is empty. addFirst(10) -> [10]. addLast(20) -> [10, 20]. addFirst(5) -> [5, 10, 20]. removeLast() removes 20 -> [5, 10]. addLast(15) -> [5, 10, 15].

❓ Predict Output

intermediate

Result of peek operations on Dequeue

What will be the output of the following code snippet?

DSA Java

import java.util.Deque;
import java.util.LinkedList;

public class Main {
    public static void main(String[] args) {
        Deque<String> dq = new LinkedList<>();
        dq.addLast("apple");
        dq.addLast("banana");
        dq.addFirst("cherry");
        System.out.println(dq.peekFirst());
        System.out.println(dq.peekLast());
    }
}

A cherry\nbanana

B apple\nbanana

C banana\ncherry

D cherry\napple

💡 Hint

peekFirst returns front element, peekLast returns last element.

After addLast("apple") and addLast("banana"), dq is [apple, banana]. addFirst("cherry") makes it [cherry, apple, banana]. peekFirst() returns "cherry", peekLast() returns "banana".

🔧 Debug

advanced

Identify the error in Dequeue removal code

What error will the following code produce when executed?

DSA Java

import java.util.Deque;
import java.util.LinkedList;

public class Main {
    public static void main(String[] args) {
        Deque<Integer> dq = new LinkedList<>();
        dq.addLast(1);
        dq.addLast(2);
        dq.removeFirst();
        dq.removeFirst();
        dq.removeFirst();
    }
}

A No error, code runs fine

B NullPointerException

C java.util.NoSuchElementException

D IndexOutOfBoundsException

💡 Hint

What happens if you remove from an empty dequeue using removeFirst()?

After removing twice, the dequeue is empty. The third removeFirst() call throws NoSuchElementException because there is no element to remove.

❓ Predict Output

advanced

Output after mixed add and remove operations on Dequeue

What is the output of the following code?

DSA Java

import java.util.Deque;
import java.util.LinkedList;

public class Main {
    public static void main(String[] args) {
        Deque<Character> dq = new LinkedList<>();
        dq.addLast('A');
        dq.addFirst('B');
        dq.addLast('C');
        dq.removeFirst();
        dq.addFirst('D');
        dq.removeLast();
        System.out.println(dq);
    }
}

A [B, A, C]

B [D, A]

C [D, B, A]

D [A, C]

💡 Hint

Track each operation step by step carefully.

Start: []. addLast('A') -> [A]. addFirst('B') -> [B, A]. addLast('C') -> [B, A, C]. removeFirst() removes B -> [A, C]. addFirst('D') -> [D, A, C]. removeLast() removes C -> [D, A].

🧠 Conceptual

expert

Dequeue behavior under concurrent modifications

In a multi-threaded Java program using a LinkedList as a Dequeue, what issue can arise if multiple threads modify the dequeue without synchronization?

A Automatic thread safety with no issues

B Compile-time error due to thread conflicts

C Deadlock due to internal locking in LinkedList

D Data inconsistency and ConcurrentModificationException

💡 Hint

Think about thread safety of LinkedList and what happens when multiple threads modify it.

LinkedList is not thread-safe. Concurrent modifications without synchronization can cause data inconsistency and throw ConcurrentModificationException during iteration.

Practice

1. What is the main difference between a deque and a regular queue?

easy

A. Deque is slower than queue in all operations

B. Deque only allows insertion, queue only allows deletion

C. Deque allows insertion and deletion from both ends, queue only from one end

D. Deque stores elements in sorted order, queue does not

Solution

1. Step 1: Understand queue operations

   A regular queue allows insertion at the rear and deletion from the front only.

2. Step 2: Understand deque operations

   A deque (double-ended queue) allows insertion and deletion at both front and rear ends.

3. Final Answer:

   Deque allows insertion and deletion from both ends, queue only from one end -> Option C

4. Quick Check:

   Deque = Both ends operations [OK]

Hint: Deque = double-ended operations, queue = single-ended [OK]

Common Mistakes:

• Thinking deque only allows insertion
• Confusing deque with stack
• Assuming deque stores sorted data

2. Which of the following is the correct way to add an element to the front of a Java Deque named dq?

easy

A. dq.addLast(element);

B. dq.addFirst(element);

C. dq.pushBack(element);

D. dq.enqueueFront(element);

Solution

1. Step 1: Recall Java Deque methods

   Java Deque interface provides addFirst() to add element at the front.

2. Step 2: Check options for correct method

   Only dq.addFirst(element); correctly adds at front.

3. Final Answer:

   dq.addFirst(element); -> Option B

4. Quick Check:

   addFirst() adds front [OK]

Hint: Use addFirst() to add at front in Java Deque [OK]

Common Mistakes:

• Using addLast() to add at front
• Using non-existent methods like pushBack()
• Confusing enqueue with Deque methods

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

Deque<Integer> dq = new ArrayDeque<>();
dq.addLast(10);
dq.addFirst(20);
dq.addLast(30);
System.out.println(dq);

medium

A. [20, 30, 10]

B. [10, 20, 30]

C. [30, 10, 20]

D. [20, 10, 30]

Solution

1. Step 1: Trace additions to deque

   First addLast(10) adds 10 at rear: [10]. Then addFirst(20) adds 20 at front: [20, 10]. Then addLast(30) adds 30 at rear: [20, 10, 30].

2. Step 2: Print deque state

   Printing dq shows elements in order: [20, 10, 30].

3. Final Answer:

   [20, 10, 30] -> Option D

4. Quick Check:

   addFirst adds front, addLast adds rear [OK]

Hint: Track addFirst at front, addLast at rear [OK]

Common Mistakes:

• Mixing order of addFirst and addLast
• Assuming addLast adds at front
• Confusing printed order with insertion order

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

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

medium

A. removeFront() method does not exist in Deque

B. addFirst() cannot be used with LinkedList

C. Deque cannot store String elements

D. dq should be initialized with ArrayDeque, not LinkedList

Solution

1. Step 1: Check Deque methods

   Deque interface does not have a method named removeFront(). The correct method to remove from front is removeFirst().

2. Step 2: Verify other statements

   addFirst() works with LinkedList, Deque can store Strings, and LinkedList is a valid Deque implementation.

3. Final Answer:

   removeFront() method does not exist in Deque -> Option A

4. Quick Check:

   Use removeFirst() to remove front [OK]

Hint: Use removeFirst(), not removeFront() [OK]

Common Mistakes:

• Using removeFront() instead of removeFirst()
• Thinking LinkedList can't implement Deque
• Confusing method names for removal

5. Given a Deque dq initially empty, which sequence of operations will result in the deque containing [5, 10, 15] from front to rear?

hard

A. dq.addFirst(5); dq.addLast(10); dq.addLast(15);

B. dq.addLast(15); dq.addLast(10); dq.addLast(5);

C. dq.addFirst(5); dq.addFirst(10); dq.addFirst(15);

D. dq.addLast(5); dq.addLast(15); dq.addLast(10);

Solution

1. Step 1: Analyze dq.addFirst(5); dq.addLast(10); dq.addLast(15); operations

   Start empty: []. addFirst(5) -> [5]. addLast(10) -> [5, 10]. addLast(15) -> [5, 10, 15].

2. Step 2: Verify final deque order

   Deque after operations is [5, 10, 15], matching the required order.

3. Final Answer:

   dq.addFirst(5); dq.addLast(10); dq.addLast(15); -> Option A

4. Quick Check:

   addFirst front, addLast rear = correct order [OK]

Hint: Add 5 front, then 10 and 15 at rear for correct order [OK]

Common Mistakes:

• Adding all elements at front reverses order
• Adding all elements at rear but starting with wrong element
• Mixing addFirst and addLast order