Stack Implementation Using Linked List in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack in data structures?

A stack is a collection where elements are added and removed from only one end called the top. It follows Last In, First Out (LIFO) order.

beginner

Why use a linked list to implement a stack?

Using a linked list allows dynamic memory use, so the stack can grow or shrink without fixed size limits, unlike arrays.

beginner

What does the push operation do in a stack?

Push adds a new element to the top of the stack.

beginner

What does the pop operation do in a stack?

Pop removes the top element from the stack and returns it.

intermediate

In a linked list based stack, where is the top element stored?

The top element is stored at the head (start) of the linked list for easy access and updates.

Which operation adds an element to the stack?

A Push

B Pop

C Peek

D Delete

Push adds a new element to the top of the stack.

What order does a stack follow?

A Last In, First Out

B Sorted Order

C Random Order

D First In, First Out

Stack follows Last In, First Out (LIFO) order.

In a linked list stack, where is the top element located?

A Tail of the list

B Middle of the list

C Random node

D Head of the list

The top element is at the head of the linked list for quick access.

What happens when you pop from an empty stack?

A Returns the last element

B Returns null or error

C Adds a new element

D Does nothing

Popping from an empty stack returns null or an error because there is no element to remove.

Why is a linked list preferred over an array for stack implementation?

A Fixed size

B Slower access

C Dynamic size and no resizing needed

D Uses more memory always

Linked lists allow dynamic size stacks without resizing, unlike arrays.

Explain how push and pop operations work in a stack implemented using a linked list.

Think about adding and removing nodes at the start of the list.

Describe the advantages of using a linked list to implement a stack compared to using an array.

Consider how memory allocation differs between arrays and linked lists.

Practice

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

easy

A. Dynamic size, no fixed limit on number of elements

B. Faster access to middle elements

C. Uses less memory for small stacks

D. Allows random access to elements

Solution

1. Step 1: Understand stack size limitation in arrays

   Arrays have fixed size or require resizing, which can be costly.

2. Step 2: Linked list dynamic size advantage

   Linked lists grow and shrink dynamically without fixed size limits.

3. Final Answer:

   Dynamic size, no fixed limit on number of elements -> Option A

4. Quick Check:

   Linked list stacks grow dynamically [OK]

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

Common Mistakes:

• Thinking linked lists allow random access
• Confusing memory usage benefits
• Assuming arrays are always faster

2. Which of the following is the correct way to create a new node in a linked list stack in JavaScript?

easy

A. const node = Node(val, null);

B. const node = new Node(val, null);

C. const node = { value: val, next: null };

D. const node = { val: value, nextNode: null };

Solution

1. Step 1: Identify correct object property names

   Standard node uses 'value' and 'next' properties.

2. Step 2: Check JavaScript object literal syntax

   const node = { value: val, next: null }; correctly creates an object with 'value' and 'next' keys.

3. Final Answer:

   const node = { value: val, next: null }; -> Option C

4. Quick Check:

   Object with value and next keys [OK]

Hint: Node uses 'value' and 'next' keys in object [OK]

Common Mistakes:

• Using incorrect property names like 'val' or 'nextNode'
• Calling Node as a function without 'new'
• Missing 'const' or wrong syntax

3. What will be the output after pushing 10, then 20, then 30 onto an empty linked list stack and then popping once?

medium

A. Stack is empty

B. Stack top value is 10

C. Stack top value is 30

D. Stack top value is 20

Solution

1. Step 1: Push 10, 20, 30 in order

   Stack after pushes: top -> 30 -> 20 -> 10 -> null

2. Step 2: Pop once removes top (30)

   New top after pop is 20.

3. Final Answer:

   Stack top value is 20 -> Option D

4. Quick Check:

   Pop removes 30, next top is 20 [OK]

Hint: Last pushed is first popped, next top is previous push [OK]

Common Mistakes:

• Thinking pop removes bottom element
• Confusing order of pushes
• Assuming stack becomes empty after one pop

4. Given this pop method code for a linked list stack, what is the error?

pop() {
  if (!this.top) return null;
  const poppedNode = this.top;
  this.top = poppedNode.next;
  return poppedNode.value;
}

medium

A. Should check if poppedNode.next exists before assigning

B. Should decrement stack size counter after pop

C. Should set poppedNode.next to null after popping

D. No error, code works correctly

Solution

1. Step 1: Analyze pop method logic

   Pop removes top node and returns its value correctly.

2. Step 2: Check for missing stack size update

   Stack size counter is not decremented, causing size mismatch.

3. Final Answer:

   Should decrement stack size counter after pop -> Option B

4. Quick Check:

   Pop must update size counter [OK]

Hint: Always update size counter on push/pop [OK]

Common Mistakes:

• Thinking next pointer must be cleared
• Assuming code returns wrong value
• Ignoring size counter updates

5. How would you modify a linked list stack to support a peek() method that returns the top element without removing it? Choose the correct implementation.

hard

A. peek() { return this.top ? this.top.value : null; }

B. peek() { const temp = this.pop(); this.push(temp); return temp; }

C. peek() { return this.top.next ? this.top.next.value : null; }

D. peek() { this.top = this.top.next; return this.top.value; }

Solution

1. Step 1: Understand peek method purpose

   Peek returns top value without changing stack.

2. Step 2: Evaluate each option

   peek() { return this.top ? this.top.value : null; } returns top value safely without modifying stack; others modify or access wrong nodes.

3. Final Answer:

   peek() { return this.top ? this.top.value : null; } -> Option A

4. Quick Check:

   Peek returns top value without removal [OK]

Hint: Peek reads top value, no pop or pointer changes [OK]

Common Mistakes:

• Using pop inside peek causing removal
• Accessing next node instead of top
• Changing top pointer in peek