Peek Top Element of Stack in DSA Typescript - 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.

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

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Stack {
  private items: number[] = [];

  peek(): number | undefined {
    return this.items[this.items.length - 1];
  }
}
    

This code returns the top element of the stack without removing it.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Accessing the last element of an array.
• How many times: Exactly once per peek call.

How Execution Grows With Input

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

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

Pattern observation: The time stays the same even if the stack grows.

Final Time Complexity

Time Complexity: O(1)

This means peeking the top element takes a constant amount of time regardless of stack size.

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 element directly, so it does not depend on stack size.

Interview Connect

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

Self-Check

"What if the stack was implemented using 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 supports push (add), pop (remove), and peek (view) operations.

2. Step 2: Define peek operation

   Peek returns the top element without removing it, unlike pop which removes it.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   Peek = view top element only [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

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

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

easy

A. stack[stack.length - 1]

B. stack.peek()

C. stack.pop()

D. stack.top()

Solution

1. Step 1: Recall array indexing for top element

   The top element of a stack implemented as an array is at index length - 1.

2. Step 2: Identify correct syntax

   Accessing stack[stack.length - 1] returns the top element without removing it.

3. Final Answer:

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

4. Quick Check:

   Array top = stack[length - 1] [OK]

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

Common Mistakes:

• Using pop() which removes element
• Using non-existent peek() method on array
• Using top() which is not a standard method

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

const stack: number[] = [];
stack.push(10);
stack.push(20);
stack.push(30);
console.log(stack[stack.length - 1]);

medium

A. 10

B. 20

C. 30

D. undefined

Solution

1. Step 1: Trace stack push operations

   Elements pushed in order: 10, then 20, then 30. Stack is [10, 20, 30].

2. Step 2: Access top element using stack[stack.length - 1]

   stack.length is 3, so index 2 is top element which is 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Top element after pushes = 30 [OK]

Hint: Last pushed element is top [OK]

Common Mistakes:

• Confusing first pushed element as top
• Using pop() instead of peek
• Off-by-one error in index

4. Identify the error in the following TypeScript code that tries to peek the top element of a stack implemented as an array:

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

medium

A. Missing return type in peek function

B. stack array is not initialized properly

C. peek function should return stack[0]

D. Using pop() removes the top element instead of just peeking

Solution

1. Step 1: Understand pop() behavior

   pop() removes and returns the top element, modifying the stack.

2. Step 2: Identify peek operation requirement

   Peek should return the top element without removing it, so pop() is incorrect here.

3. Final Answer:

   Using pop() removes the top element instead of just peeking -> Option D

4. Quick Check:

   pop() removes element, peek should not [OK]

Hint: Peek must not remove element; pop() does [OK]

Common Mistakes:

• Using pop() instead of accessing last element
• Returning stack[0] which is bottom element
• Ignoring side effects of pop()

5. You have a stack implemented as an array stack. You want to safely peek the top element but return null if the stack is empty to avoid errors. Which TypeScript code snippet correctly does this?

hard

A. return stack.pop() || null;

B. return stack.length ? stack[stack.length - 1] : null;

C. return stack[0] || null;

D. return stack[stack.length];

Solution

1. Step 1: Check if stack is empty

   Use stack.length to verify if stack has elements.

2. Step 2: Return top element or null

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

3. Final Answer:

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

4. Quick Check:

   Safe peek with empty check = return stack.length ? stack[stack.length - 1] : null; [OK]

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

Common Mistakes:

• Using pop() which removes element
• Accessing stack[0] which is bottom element
• Accessing stack[stack.length] which is undefined