Stack Implementation Using Linked List in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the top node of the stack.

DSA Java

private Node [1];

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

Atop

Bhead

Cstart

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head' or 'root' which are more common in linked lists but not typical for stack top.

Using variable names that do not clearly indicate the stack's top.

The top of the stack is usually referenced by a variable named 'top' in linked list stack implementations.

2fill in blank

medium

Complete the push method to add a new node at the top of the stack.

DSA Java

public void push(int data) {
    Node newNode = new Node(data);
    newNode.next = [1];
    top = newNode;
}

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

Anull

Btop

CnewNode

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next to null, which breaks the chain.

Assigning newNode.next to newNode itself, causing a cycle.

The new node's next should point to the current top node before updating top to newNode.

3fill in blank

hard

Fix the error in the pop method to correctly remove and return the top element.

DSA Java

public int pop() {
    if (top == null) {
        throw new EmptyStackException();
    }
    int value = top.data;
    top = [1];
    return value;
}

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

Anull

Btop

Ctop.next

Dtop.data

Attempts:

3 left

💡 Hint

Common Mistakes

Setting top to itself, causing no removal.

Setting top to null always, which empties the stack incorrectly.

To remove the top node, update top to point to the next node in the stack.

4fill in blank

hard

Fill both blanks to check if the stack is empty and return the correct boolean.

DSA Java

public boolean isEmpty() {
    return [1] == [2];
}

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

Atop

Bnull

Chead

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing head or size which are not used here.

Using incorrect comparison operators.

The stack is empty if the top node is null.

5fill in blank

hard

Fill all three blanks to implement the peek method that returns the top element without removing it.

DSA Java

public int peek() {
    if ([1] == [2]) {
        throw new EmptyStackException();
    }
    return [3].data;
}

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

Atop

Bnull

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using head instead of top.

Returning data after removing the node.

Peek checks if top is null to throw exception, then returns top.data without removing it.

Practice

1. What is the main advantage of implementing a stack using a linked list instead of an array?

easy

A. It uses less memory than an array

B. The stack can grow dynamically without a fixed size limit

C. It allows random access to elements

D. It is easier to sort the elements

Solution

1. Step 1: Understand stack size limitation in arrays

   Arrays have a fixed size, so stacks implemented with arrays have a maximum capacity.

2. Step 2: Linked list dynamic size advantage

   Linked lists can grow or shrink dynamically by adding or removing nodes, so the stack size is limited only by memory.

3. Final Answer:

   The stack can grow dynamically without a fixed size limit -> Option B

4. Quick Check:

   Dynamic size = Advantage of linked list stack [OK]

Hint: Linked list stacks grow dynamically, arrays don't [OK]

Common Mistakes:

• Thinking linked list uses less memory always
• Assuming stacks allow random access
• Confusing sorting with stack operations

2. Which of the following is the correct way to declare a node class for a linked list stack in Java?

easy

A. class Node { int data; int next; }

B. class Node { int data; Node prev; }

C. class Node { Node data; Node next; }

D. class Node { int data; Node next; }

Solution

1. Step 1: Identify node structure for linked list stack

   A node should store data and a reference to the next node.

2. Step 2: Check each option

   class Node { int data; Node next; } correctly uses 'int data' and 'Node next'. class Node { int data; Node prev; } uses 'prev' which is for doubly linked lists. class Node { int data; int next; } wrongly uses 'int next'. class Node { Node data; Node next; } wrongly uses 'Node data'.

3. Final Answer:

   class Node { int data; Node next; } -> Option D

4. Quick Check:

   Node stores data + next pointer [OK]

Hint: Node must have data and next pointer for stack [OK]

Common Mistakes:

• Using 'prev' pointer in singly linked list
• Declaring next as int instead of Node
• Using Node type for data field

3. What will be the output after the following operations on an empty linked list stack?

push(10); push(20); push(30); pop(); push(40);

Print the stack from top to bottom.

medium

A. 40 -> 30 -> 20 -> 10 -> null

B. 30 -> 20 -> 10 -> null

C. 40 -> 20 -> 10 -> null

D. 20 -> 10 -> null

Solution

1. Step 1: Trace push and pop operations

   push(10) -> stack: 10
   push(20) -> stack: 20 -> 10
   push(30) -> stack: 30 -> 20 -> 10
   pop() removes 30 -> stack: 20 -> 10
   push(40) -> stack: 40 -> 20 -> 10

2. Step 2: Write final stack order

   Top is 40, then 20, then 10, then null.

3. Final Answer:

   40 -> 20 -> 10 -> null -> Option C

4. Quick Check:

   Stack top after ops = 40 -> 20 -> 10 [OK]

Hint: Push adds top, pop removes top [OK]

Common Mistakes:

• Forgetting pop removes top element
• Appending instead of prepending on push
• Confusing order of elements after operations

4. Identify the error in the following pop method of a linked list stack:

public int pop() {
  if (top == null) {
    System.out.println("Stack Underflow");
  }
  int value = top.data;
  top = top.next;
  return value;
}

medium

A. It does not handle empty stack properly before accessing top.data

B. It should return void instead of int

C. It should update top.next instead of top

D. It should print "Stack Overflow" instead of "Stack Underflow"

Solution

1. Step 1: Check empty stack condition handling

   The method prints "Stack Underflow" but continues to access top.data without stopping or returning.

2. Step 2: Consequence of missing return or exception

   Accessing top.data when top is null causes NullPointerException. Proper handling requires returning or throwing after printing.

3. Final Answer:

   It does not handle empty stack properly before accessing top.data -> Option A

4. Quick Check:

   Null check must prevent access [OK]

Hint: Check for empty stack before accessing data [OK]

Common Mistakes:

• Not returning or throwing after underflow message
• Confusing overflow with underflow
• Updating wrong pointer in pop

5. You want to implement a method reverseStack() that reverses the elements of a linked list stack without using extra data structures. Which approach is best?

hard

A. Use recursion to pop all elements and push them back in reverse order

B. Traverse the linked list and swap data of nodes pairwise

C. Create a new linked list and copy elements in reverse order

D. Use a queue to dequeue and enqueue elements to reverse

Solution

1. Step 1: Understand constraints of no extra data structures

   Options C and D use extra structures (new list or queue), so they are invalid.

2. Step 2: Evaluate recursion and swapping

   Swapping data pairwise is complex and error-prone for linked list stack. Recursion can pop all elements and push back in reverse order naturally.

3. Final Answer:

   Use recursion to pop all elements and push them back in reverse order -> Option A

4. Quick Check:

   Recursion reverses stack without extra storage [OK]

Hint: Recursion can reverse stack without extra space [OK]

Common Mistakes:

• Using extra data structures despite constraints
• Trying to swap node data instead of nodes
• Confusing queue operations with stack reversal