Peek Front Element of Queue in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after peeking the front element?

Consider a queue implemented using an array. What will be the output of peeking the front element after the given operations?

DSA Typescript

class Queue {
  private items: number[] = [];
  enqueue(element: number) {
    this.items.push(element);
  }
  dequeue() {
    return this.items.shift();
  }
  peek() {
    return this.items[0];
  }
}

const queue = new Queue();
queue.enqueue(10);
queue.enqueue(20);
queue.enqueue(30);
queue.dequeue();
console.log(queue.peek());

A undefined

B 20

C 30

D 10

💡 Hint

Remember that dequeue removes the front element before peeking.

Initially, the queue has [10, 20, 30]. After dequeue, 10 is removed, so the front is 20. Peeking returns 20.

❓ Predict Output

intermediate

What does peek return on an empty queue?

What will be the output of peeking the front element of an empty queue implemented with an array?

DSA Typescript

class Queue {
  private items: number[] = [];
  peek() {
    return this.items[0];
  }
}

const queue = new Queue();
console.log(queue.peek());

A Throws an error

B null

C 0

D undefined

💡 Hint

Accessing the first element of an empty array returns what?

Accessing index 0 of an empty array returns undefined in JavaScript/TypeScript.

❓ Predict Output

advanced

What is the output after multiple enqueues and peeks?

Given the following queue operations, what is printed after the last peek?

DSA Typescript

class Queue {
  private items: number[] = [];
  enqueue(element: number) {
    this.items.push(element);
  }
  peek() {
    return this.items[0];
  }
}

const queue = new Queue();
queue.enqueue(5);
queue.enqueue(15);
queue.enqueue(25);
console.log(queue.peek());
queue.enqueue(35);
console.log(queue.peek());

A 5\n5

B 5\n35

C 15\n15

D 25\n35

💡 Hint

Peek always returns the first element without removing it.

Peek returns the first element which is 5 both times since no dequeue happened.

❓ Predict Output

advanced

What error occurs when peeking after clearing the queue?

What will be the output or error after peeking the front element of a queue cleared by removing all elements?

DSA Typescript

class Queue {
  private items: number[] = [];
  enqueue(element: number) {
    this.items.push(element);
  }
  dequeue() {
    return this.items.shift();
  }
  peek() {
    return this.items[0];
  }
}

const queue = new Queue();
queue.enqueue(1);
queue.enqueue(2);
queue.dequeue();
queue.dequeue();
console.log(queue.peek());

A null

B Throws TypeError

C undefined

D 0

💡 Hint

What happens when you access the first element of an empty array?

After removing all elements, the array is empty and accessing index 0 returns undefined.

🧠 Conceptual

expert

Why is peeking the front element of a queue O(1) time complexity?

Choose the best explanation why peeking the front element of a queue is a constant time operation.

A Because the front element is always at a fixed position accessible directly without iteration.

B Because peeking requires traversing the entire queue to find the front element.

C Because peeking sorts the queue first to find the smallest element.

D Because peeking involves removing all elements before accessing the front.

💡 Hint

Think about how queues store their front element.

Queues keep track of the front element directly, so accessing it does not require looping.

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