Peek Front Element of Queue in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It shows the front element of the queue without removing it, so you can see which item will be removed next.

beginner

In a queue, which end is the 'front'?

The front is the end where elements are removed (dequeued). It is the oldest element in the queue.

beginner

TypeScript code snippet: How to peek the front element of a queue implemented as an array named 'queue'?

const front = queue.length > 0 ? queue[0] : undefined; // front holds the first element or undefined if empty

beginner

Why is peek useful in a queue?

Peek lets you check the next item to be removed without changing the queue, useful for decision making or preview.

beginner

What happens if you peek an empty queue?

You get 'undefined' or a special value indicating the queue is empty, because there is no front element.

What does the peek operation return in a queue?

A The front element without removing it

B The last element and removes it

C All elements in the queue

D Removes the front element

Peek returns the front element without removing it, so you can see the next item to be dequeued.

If a queue is empty, what should peek return?

A The first element

B Undefined or null

C The last element

D An error always

Peek returns undefined or null if the queue is empty, indicating no front element exists.

In a queue implemented as an array, which index holds the front element?

A 0

B Length - 1

C Middle index

D Random index

The front element is at index 0 in an array-based queue.

Which operation does NOT change the queue?

A Enqueue

B Dequeue

C Peek

D Clear

Peek only reads the front element without removing it, so the queue stays the same.

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

A O(n^2)

B O(n)

C O(log n)

D O(1)

Peek accesses the front element directly by index 0, so it runs in constant time O(1).

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

Think about checking the next item without changing the queue.

Describe how you would implement peek in a queue using an array in TypeScript.

Remember the front is at the start of the array.

Practice

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

easy

A. Returns the front element without removing it

B. Removes the front element from the queue

C. Adds an element to the back of the queue

D. Checks if the queue is empty

Solution

1. Step 1: Understand the peek operation

   The peek operation looks at the front element of the queue but does not remove it.

2. Step 2: Compare with other operations

   Removing is done by dequeue, adding by enqueue, and checking empty is a separate check.

3. Final Answer:

   Returns the front element without removing it -> Option A

4. Quick Check:

   Peek = front element without removal [OK]

Hint: Peek shows front without removal [OK]

Common Mistakes:

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

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

easy

A. const front = queue.pop();

B. const front = queue.shift();

C. const front = queue.push();

D. const front = queue[0];

Solution

1. Step 1: Understand array methods for queue

   In a queue using an array, the front element is at index 0.

2. Step 2: Identify correct syntax for peek

   Accessing queue[0] returns the front without removing it. pop() removes from end, shift() removes from front, push() adds to end.

3. Final Answer:

   const front = queue[0]; -> Option D

4. Quick Check:

   Peek = queue[0] [OK]

Hint: Peek front is queue[0], not pop or shift [OK]

Common Mistakes:

• Using pop() which removes last element
• Using shift() which removes front element
• Using push() which adds element

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

const queue: number[] = [10, 20, 30];
const front = queue[0];
console.log(front);
console.log(queue);

medium

A. undefined
[10, 20, 30]

B. 10
[20, 30]

C. 10
[10, 20, 30]

D. 30
[10, 20, 30]

Solution

1. Step 1: Peek front element from queue array

   Accessing queue[0] returns 10, the first element.

2. Step 2: Check if queue is modified

   Since we only accessed queue[0], the array remains unchanged as [10, 20, 30].

3. Final Answer:

   10 [10, 20, 30] -> Option C

4. Quick Check:

   Peek returns 10, queue unchanged [OK]

Hint: Peek does not remove, queue stays same [OK]

Common Mistakes:

• Assuming peek removes element
• Confusing front with last element
• Expecting undefined for valid index

4. Identify the error in this TypeScript code that tries to peek the front element of a queue:

class Queue {
  items: number[] = [];
  peek() {
    return this.items.pop();
  }
}

medium

A. pop() removes last element, should use items[0]

B. pop() is correct for peek operation

C. items array should be private

D. peek method should return void

Solution

1. Step 1: Understand pop() behavior

   pop() removes and returns the last element, not the front.

2. Step 2: Correct way to peek front

   To peek front, return this.items[0] without removing it.

3. Final Answer:

   pop() removes last element, should use items[0] -> Option A

4. Quick Check:

   pop() ≠ peek front [OK]

Hint: pop() removes last, peek front is items[0] [OK]

Common Mistakes:

• Using pop() for peek
• Not returning front element correctly
• Ignoring array indexing

5. You have a queue implemented as a linked list with nodes having value and next. How do you peek the front element safely when the queue might be empty?

hard

A. Return head.value directly without checks

B. Check if head is null; if not, return head.value, else return null

C. Traverse to the last node and return its value

D. Remove the head node and return its value

Solution

1. Step 1: Understand linked list queue front

   The front element is at the head node of the linked list.

2. Step 2: Handle empty queue case

   If head is null, queue is empty, so return null or undefined safely.

3. Step 3: Return front value safely

   If head exists, return head.value without removing the node.

4. Final Answer:

   Check if head is null; if not, return head.value, else return null -> Option B

5. Quick Check:

   Peek front safely = check head then return value [OK]

Hint: Check head null before peek to avoid errors [OK]

Common Mistakes:

• Not checking for empty queue
• Returning value without null check
• Removing node instead of peeking