Peek Top Element of Stack in DSA Javascript - Time & Space Complexity

Time Complexity: Peek Top Element of Stack

O(1)

Understanding Time Complexity

We want to understand how long it takes to look at the top item of a stack without removing it.

How does the time needed change as the stack grows bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function peek(stack) {
  if (stack.length === 0) {
    return null; // stack is empty
  }
  return stack[stack.length - 1];
}
    

This code returns the top element of the stack if it exists, otherwise returns null.

Identify Repeating Operations

• Primary operation: Accessing the last element of the array (stack).
• How many times: Exactly once per peek call, no loops or recursion.

How Execution Grows With Input

Looking at the top element takes the same amount of time no matter how big the stack is.

Input Size (n)	Approx. Operations
10	1
100	1
1000	1

Pattern observation: The time stays constant as the stack grows.

Final Time Complexity

Time Complexity: O(1)

This means peeking the top element takes the same short time no matter how many items are in the stack.

Common Mistake

[X] Wrong: "Peeking takes longer if the stack is bigger because it has to check many elements."

[OK] Correct: Peeking only looks at the last item directly without scanning the stack, so size does not affect time.

Interview Connect

Knowing that peek is a quick, constant-time operation helps you explain stack behavior clearly and confidently in interviews.

Self-Check

"What if the stack was implemented as a linked list instead of an array? How would the time complexity of peek change?"

Practice

1. What does the peek operation do in a stack?

easy

A. Returns the top element without removing it

B. Removes the top element and returns it

C. Adds a new element to the top

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack operations

   A stack stores elements with last-in, first-out order. Peek shows the top element without removing it.

2. Step 2: Differentiate peek from pop

   Pop removes and returns the top element, peek only returns it without removal.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   Peek = top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing peek with pop operation
• Thinking peek adds elements
• Assuming peek checks emptiness

2. Which of the following is the correct syntax to peek the top element of a stack implemented as an array named stack in JavaScript?

easy

A. stack.peek()

B. stack.pop()

C. stack[stack.length - 1]

D. stack.top()

Solution

1. Step 1: Understand JavaScript array indexing

   JavaScript arrays use zero-based indexing. The last element is at index length - 1.

2. Step 2: Identify correct peek syntax

   To peek, access the last element without removing it: stack[stack.length - 1]. stack.pop() removes the element, stack.peek() and stack.top() are not default methods.

3. Final Answer:

   stack[stack.length - 1] -> Option C

4. Quick Check:

   Peek = stack[stack.length - 1] [OK]

Hint: Use stack[stack.length - 1] to peek top [OK]

Common Mistakes:

• Using stack.pop() which removes element
• Using non-existent methods like peek() or top()
• Accessing stack[stack.length] which is undefined

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

const stack = [10, 20, 30];
const top = stack[stack.length - 1];
console.log(top);

medium

A. 10

B. 30

C. 20

D. undefined

Solution

1. Step 1: Identify the stack elements and length

   The stack array is [10, 20, 30]. Its length is 3.

2. Step 2: Calculate the top element index and value

   stack.length - 1 = 3 - 1 = 2. stack[2] is 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   Top element = 30 [OK]

Hint: Top element is last array item: stack[length-1] [OK]

Common Mistakes:

• Using stack[stack.length] which is undefined
• Confusing first element with top element
• Printing stack.pop() instead of peek

4. Find the error in this code snippet that tries to peek the top element of a stack:

const stack = [5, 15, 25];
function peek() {
  return stack[stack.length];
}
console.log(peek());

medium

A. stack.length should be stack.length - 1

B. stack is not defined

C. peek function should remove the element

D. console.log is missing parentheses

Solution

1. Step 1: Analyze the peek function index

   stack.length returns the number of elements, but array indices go from 0 to length - 1.

2. Step 2: Correct the index to access top element

   Use stack.length - 1 to get the last element. Using stack.length returns undefined.

3. Final Answer:

   stack.length should be stack.length - 1 -> Option A

4. Quick Check:

   Index = length - 1 for top element [OK]

Hint: Use length - 1 to access last element [OK]

Common Mistakes:

• Using stack.length as index causing undefined
• Thinking peek removes element
• Missing parentheses in console.log (not in this code)

5. Given a stack implemented as an array, how can you safely peek the top element without causing an error if the stack is empty?

function safePeek(stack) {
  // Your code here
}

hard

A. return stack[stack.length];

B. return stack[0];

C. return stack.pop();

D. return stack.length > 0 ? stack[stack.length - 1] : null;

Solution

1. Step 1: Check if stack is empty

   Before accessing the top element, verify stack length is greater than zero to avoid undefined access.

2. Step 2: Return top element or null safely

   If stack is not empty, return stack[stack.length - 1], else return null to indicate empty stack.

3. Final Answer:

   return stack.length > 0 ? stack[stack.length - 1] : null; -> Option D

4. Quick Check:

   Safe peek uses length check and conditional [OK]

Hint: Check length > 0 before peek to avoid errors [OK]

Common Mistakes:

• Accessing stack[stack.length] causing undefined
• Using pop which removes element
• Returning stack[0] which is bottom, not top