Stack Implementation Using Linked List in DSA Javascript - Time & Space Complexity

Time Complexity: Stack Implementation Using Linked List

O(1)

Understanding Time Complexity

We want to understand how fast stack operations run when using a linked list.

How does the time to add or remove items change as the stack grows?

Scenario Under Consideration

Analyze the time complexity of the following stack operations using a linked list.

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class Stack {
  constructor() {
    this.top = null;
  }

  push(value) {
    const newNode = new Node(value);
    newNode.next = this.top;
    this.top = newNode;
  }

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

This code adds (push) and removes (pop) items at the top of the stack using a linked list.

Identify Repeating Operations

Look for loops or repeated steps in push and pop.

• Primary operation: Assigning pointers to add or remove the top node.
• How many times: Each push or pop does a fixed number of steps, no loops.

How Execution Grows With Input

Each push or pop takes the same small number of steps no matter how big the stack is.

Input Size (n)	Approx. Operations
10	3-4 steps per push or pop
100	3-4 steps per push or pop
1000	3-4 steps per push or pop

Pattern observation: The time stays the same no matter how many items are in the stack.

Final Time Complexity

Time Complexity: O(1)

This means adding or removing an item takes the same short time regardless of stack size.

Common Mistake

[X] Wrong: "Pop or push takes longer as the stack grows because it has more items."

[OK] Correct: Each operation only changes the top pointer, so it does not look at all items.

Interview Connect

Knowing that stack operations are quick and constant time helps you explain efficient data handling in interviews.

Self-Check

"What if we changed the stack to use an array instead of a linked list? How would the time complexity change?"

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