Peek Top Element of Stack in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'peek' operation do in a stack?

The 'peek' operation returns the top element of the stack without removing it, allowing you to see what is on top.

beginner

In a stack, what happens if you try to peek when the stack is empty?

Peeking an empty stack usually returns a special value like null or undefined, or throws an error, because there is no top element to show.

intermediate

TypeScript code snippet: How do you define a peek method for a stack class?

A peek method returns the last element in the internal array without removing it, for example:

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

beginner

Why is peek useful compared to pop in stack operations?

Peek lets you see the top element without changing the stack, while pop removes the top element. Peek is useful when you want to check the top without losing data.

beginner

What is the time complexity of the peek operation in a stack implemented with an array?

The peek operation runs in O(1) time because it directly accesses the last element of the array without looping.

What does the peek operation return in a stack?

A The bottom element

B Removes and returns the top element

C The top element without removing it

D The size of the stack

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

If a stack is empty, what should peek return or do?

A Return the top element

B Return null or undefined, or throw an error

C Add a new element

D Remove the bottom element

Peeking an empty stack has no top element, so it returns null/undefined or throws an error.

What is the time complexity of peek in a stack implemented with an array?

A O(1)

B O(log n)

C O(n)

D O(n^2)

Peek accesses the last element directly, so it runs in constant time O(1).

Which stack operation removes the top element?

A Pop

B Push

C Peek and Pop

D Peek

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

In TypeScript, how do you access the top element of a stack stored as an array named 'items'?

A items.pop()

B items[0]

C items.length

D items[items.length - 1]

The top element is the last element in the array, accessed by items[items.length - 1].

Explain how the peek operation works in a stack and why it is useful.

Think about checking the top book on a stack without taking it off.

Write a simple TypeScript peek method for a stack implemented with an array.

Use array indexing to get the last element.

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