Traversal Forward and Backward in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to move the iterator forward by one element.

DSA Java

ListIterator<String> iterator = list.listIterator();
String nextElement = iterator.[1]();

Drag options to blanks, or click blank then click option'

Anext

BhasNext

Cprevious

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using previous() instead of next()

Using hasNext() which only checks if next element exists

The next() method moves the iterator forward and returns the next element.

2fill in blank

medium

Complete the code to check if there is a previous element before moving backward.

DSA Java

if (iterator.[1]()) {
    String prevElement = iterator.previous();
}

Drag options to blanks, or click blank then click option'

Aprevious

BhasNext

ChasPrevious

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using hasNext() which checks forward movement

Calling previous() without checking hasPrevious()

The hasPrevious() method checks if there is an element before the current position.

3fill in blank

hard

Fix the error in the code to correctly traverse backward through the list.

DSA Java

while (iterator.[1]()) {
    System.out.println(iterator.previous());
}

Drag options to blanks, or click blank then click option'

AhasPrevious

Bprevious

Cnext

DhasNext

Attempts:

3 left

💡 Hint

Common Mistakes

Using hasNext() in backward traversal loop

Using next() inside backward traversal loop

To move backward, use hasPrevious() in the loop condition.

4fill in blank

hard

Fill both blanks to create a loop that prints all elements forward and then backward.

DSA Java

while (iterator.[1]()) {
    System.out.print(iterator.next() + " ");
}
while (iterator.[2]()) {
    System.out.print(iterator.previous() + " ");
}

Drag options to blanks, or click blank then click option'

AhasNext

BhasPrevious

Cnext

Dprevious

Attempts:

3 left

💡 Hint

Common Mistakes

Using hasPrevious() in the first loop

Using hasNext() in the second loop

Use hasNext() to move forward and hasPrevious() to move backward safely.

5fill in blank

hard

Fill all three blanks to create a loop that moves forward, prints elements, then moves backward printing elements again.

DSA Java

while (iterator.[1]()) {
    System.out.print(iterator.[2]() + " ");
}
while (iterator.[3]()) {
    System.out.print(iterator.previous() + " ");
}

Drag options to blanks, or click blank then click option'

AhasNext

Bnext

ChasPrevious

Dprevious

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing next() and previous() calls incorrectly

Not checking hasPrevious() before moving backward

The first loop uses hasNext() and next() to move forward and print. The second loop uses hasPrevious() to move backward safely.

Practice

1. What does forward traversal mean in the context of a linked list?

easy

A. Skipping every other node while visiting

B. Visiting nodes from the tail to the head in reverse order

C. Visiting nodes from the head to the tail in order

D. Visiting nodes randomly without order

Solution

1. Step 1: Understand linked list structure

   A linked list has nodes connected from head to tail in one direction.

2. Step 2: Define forward traversal

   Forward traversal means visiting nodes starting at the head and moving to the tail in order.

3. Final Answer:

   Visiting nodes from the head to the tail in order -> Option C

4. Quick Check:

   Forward traversal = head to tail [OK]

Hint: Forward means start at head and go next nodes [OK]

Common Mistakes:

• Confusing forward with backward traversal
• Thinking traversal is random
• Assuming skipping nodes is traversal

2. Which Java code snippet correctly traverses a singly linked list forward?

easy

A. for(Node curr = tail; curr != null; curr = curr.prev) { System.out.print(curr.data + " "); }

B. for(Node curr = head; curr != null; curr = curr.next) { System.out.print(curr.data + " "); }

C. while(head != null) { System.out.print(head.data + " "); head = head.prev; }

D. for(Node curr = head; curr != null; curr = curr.prev) { System.out.print(curr.data + " "); }

Solution

1. Step 1: Identify forward traversal in singly linked list

   Forward traversal uses the next pointer starting from head.

2. Step 2: Check code correctness

   for(Node curr = head; curr != null; curr = curr.next) { System.out.print(curr.data + " "); } uses curr = head; curr != null; curr = curr.next which is correct for forward traversal.

3. Final Answer:

   for(Node curr = head; curr != null; curr = curr.next) { System.out.print(curr.data + " "); } -> Option B

4. Quick Check:

   Use next pointer from head for forward traversal [OK]

Hint: Use curr = curr.next starting at head for forward traversal [OK]

Common Mistakes:

• Using prev pointer in singly linked list
• Starting from tail in singly linked list
• Using wrong loop conditions

3. What is the output of this Java code for a doubly linked list with nodes 1 <-> 2 <-> 3 when traversed backward from tail?

Node tail = thirdNode; // node with data 3
Node curr = tail;
while(curr != null) {
    System.out.print(curr.data + " ");
    curr = curr.prev;
}

medium

A. 3 2 1

B. 1 2 3

C. 3 1 2

D. 2 3 1

Solution

1. Step 1: Understand doubly linked list backward traversal

   Backward traversal starts at tail and moves using prev pointer to head.

2. Step 2: Trace the loop output

   Starting at node 3, print 3, move to 2, print 2, move to 1, print 1, then stop.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Backward traversal prints 3 2 1 [OK]

Hint: Backward traversal prints from tail to head using prev [OK]

Common Mistakes:

• Confusing forward and backward traversal
• Printing nodes in wrong order
• Using next pointer instead of prev

4. Identify the error in this code snippet that tries to traverse a doubly linked list backward:

Node curr = head;
while(curr != null) {
    System.out.print(curr.data + " ");
    curr = curr.prev;
}

medium

A. Loop condition should be curr.next != null

B. Using next pointer instead of prev pointer

C. Missing initialization of curr

D. Starting traversal from head but using prev pointer causes null pointer error

Solution

1. Step 1: Analyze traversal start and direction

   Traversal starts at head but moves using prev pointer which is null at head.

2. Step 2: Identify error cause

   Using prev from head causes immediate null pointer or no traversal backward possible.

3. Final Answer:

   Starting traversal from head but using prev pointer causes null pointer error -> Option D

4. Quick Check:

   Backward traversal must start at tail, not head [OK]

Hint: Backward traversal must start at tail node [OK]

Common Mistakes:

• Starting backward traversal at head
• Confusing prev and next pointers
• Incorrect loop conditions

5. Given a doubly linked list, how can you print its elements forward and then backward without using extra space? Choose the best approach.

hard

A. Traverse forward from head to tail printing nodes, then traverse backward from tail to head printing nodes

B. Traverse forward twice from head to tail printing nodes both times

C. Traverse backward twice from tail to head printing nodes both times

D. Store all nodes in an array then print forward and backward from array

Solution

1. Step 1: Understand traversal without extra space

   Using the doubly linked list pointers allows forward and backward traversal without extra storage.

2. Step 2: Apply forward then backward traversal

   First traverse from head to tail printing nodes, then from tail to head printing nodes using prev pointers.

3. Final Answer:

   Traverse forward from head to tail printing nodes, then traverse backward from tail to head printing nodes -> Option A

4. Quick Check:

   Use head and tail pointers for forward and backward traversal [OK]

Hint: Use head for forward and tail for backward traversal [OK]

Common Mistakes:

• Using extra space unnecessarily
• Traversing only forward twice
• Confusing traversal directions