Peek Top Element of Stack in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the top element of the stack without removing it.

DSA Typescript

peek(): number | undefined {
  if (this.items.length === 0) {
    return undefined;
  }
  return this.items[1];
}

Drag options to blanks, or click blank then click option'

A[this.items.length - 1]

B.shift()

C.pop()

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using .pop() removes the element, but peek should not remove it.

Using [0] accesses the bottom of the stack, not the top.

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

2fill in blank

medium

Complete the code to check if the stack is empty before peeking.

DSA Typescript

peek(): number | undefined {
  if (this.items.length[1] 0) {
    return undefined;
  }
  return this.items[this.items.length - 1];
}

Drag options to blanks, or click blank then click option'

A!== 0

B> 0

C< 0

D=== 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using > 0 checks if stack is not empty, which is opposite of what we want.

Using !== 0 checks if the stack is not empty, which is the opposite of what we want.

We check if the stack length is exactly zero to know if it is empty.

3fill in blank

hard

Fix the error in the peek method to correctly handle empty stack.

DSA Typescript

peek(): number | undefined {
  if (this.items.length === 0) {
    return [1];
  }
  return this.items[this.items.length - 1];
}

Drag options to blanks, or click blank then click option'

Anull

B0

Cundefined

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 or -1 can be confusing as they might be valid stack values.

Returning null is valid but less idiomatic in TypeScript for missing values.

Returning undefined clearly indicates no element is present without using a number.

4fill in blank

hard

Fill both blanks to create a peek method that returns the top element or undefined if empty.

DSA Typescript

peek(): number | undefined {
  return this.items.length [1] 0 ? [2] : this.items[this.items.length - 1];
}

Drag options to blanks, or click blank then click option'

A===

Bundefined

Cnull

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using !== 0 reverses the condition and breaks logic.

Returning null instead of undefined is less idiomatic.

If the length is exactly zero, return undefined, else return the last element.

5fill in blank

hard

Fill all three blanks to implement peek with early return for empty stack and correct top element access.

DSA Typescript

peek(): number | undefined {
  if (this.items.length [1] 0) {
    return [2];
  }
  return this.items[3];
}

Drag options to blanks, or click blank then click option'

A===

Bundefined

C[this.items.length - 1]

D[this.items.length]

Attempts:

3 left

💡 Hint

Common Mistakes

Using !== 0 in the if condition causes wrong logic.

Accessing index length returns undefined.

Check if length is exactly zero, return undefined, else return last element at index length - 1.

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