Dynamic Stack Using Resizable Array in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the top of the stack.

DSA Javascript

push(element) {
  this.stack[this.top] = element;
  this.top = this.top [1] 1;
}

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' to update the top index.

Forgetting to update the top index after push.

We increase the top index by 1 after adding an element to point to the next empty position.

2fill in blank

medium

Complete the code to check if the stack is empty.

DSA Javascript

isEmpty() {
  return this.top [1] 0;
}

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

A>

B==

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' which returns true when stack is not empty.

Using '>' or '<' which do not correctly check emptiness.

The stack is empty when top index is exactly 0.

3fill in blank

hard

Fix the error in the pop method to correctly remove and return the top element.

DSA Javascript

pop() {
  if (this.isEmpty()) {
    return null;
  }
  this.top = this.top [1] 1;
  return this.stack[this.top];
}

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Increasing top index instead of decreasing it.

Returning stack element before updating top index.

We decrease the top index by 1 before returning the element to remove the top element.

4fill in blank

hard

Fill both blanks to resize the stack array when full.

DSA Javascript

resize() {
  let newSize = this.stack.length [1] 2;
  let newStack = new Array(newSize);
  for (let i = 0; i [2] this.stack.length; i++) {
    newStack[i] = this.stack[i];
  }
  this.stack = newStack;
}

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

A*

B+

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' to increase size.

Using '>' or '>=' in loop condition causing no iteration.

We double the size by multiplying length by 2 and copy elements while i is less than original length.

5fill in blank

hard

Fill all three blanks to implement the push method with automatic resizing.

DSA Javascript

push(element) {
  if (this.top [1] this.stack.length) {
    this.resize();
  }
  this.stack[this.top] = element;
  this.top = this.top [2] 1;
  return this.stack[[3]];
}

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

A>=

B+

Cthis.top

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>=' for resize condition.

Returning element at wrong index.

Not increasing top after push.

We check if top is greater or equal to length to resize, then add element and increase top, finally return the added element at current top index.

Practice

1. What is the main advantage of using a dynamic stack implemented with a resizable array over a fixed-size array stack?

easy

A. It does not allow duplicate elements.

B. It uses less memory than a fixed-size array stack.

C. It always has a fixed capacity to avoid resizing overhead.

D. It can grow or shrink automatically as elements are added or removed.

Solution

1. Step 1: Understand fixed-size array stack limitation

   A fixed-size array stack has a set capacity and cannot grow beyond it, causing overflow if exceeded.

2. Step 2: Understand dynamic stack advantage

   A dynamic stack using a resizable array can increase its capacity automatically when full, allowing more elements without overflow.

3. Final Answer:

   It can grow or shrink automatically as elements are added or removed. -> Option D

4. Quick Check:

   Dynamic resizing = automatic growth/shrink [OK]

Hint: Dynamic stack resizes array automatically when full [OK]

Common Mistakes:

• Thinking dynamic stack uses less memory always
• Assuming fixed capacity avoids resizing overhead
• Believing dynamic stack disallows duplicates

2. Which of the following is the correct way to double the size of the internal array in a dynamic stack implemented in JavaScript?

easy

A. this.array = new Array(this.array.length * 2);

B. this.array = [...this.array, ...new Array(this.array.length)];

C. this.array.length = this.array.length + 1;

D. this.array.push(new Array(this.array.length));

Solution

1. Step 1: Understand array resizing in JavaScript

   JavaScript arrays are dynamic but to simulate resizing, we create a new larger array and copy elements.

2. Step 2: Analyze options for doubling size

   this.array = [...this.array, ...new Array(this.array.length)]; uses spread operator to copy existing elements and adds a new array of same length, effectively doubling size.

3. Final Answer:

   this.array = [...this.array, ...new Array(this.array.length)]; -> Option B

4. Quick Check:

   Spread operator copies + new array doubles size [OK]

Hint: Use spread operator to copy and extend array [OK]

Common Mistakes:

• Setting length to length + 1 only increases size by 1, not doubling
• Using push with array adds nested array, not size
• Creating new array without copying loses data

3. Consider the following JavaScript code for a dynamic stack push operation:

class DynamicStack {
  constructor() {
    this.array = [];
    this.top = -1;
  }
  push(value) {
    if (this.top + 1 === this.array.length) {
      this.array = [...this.array, ...new Array(this.array.length || 1)];
    }
    this.top++;
    this.array[this.top] = value;
  }
}
const stack = new DynamicStack();
stack.push(10);
stack.push(20);
stack.push(30);
console.log(stack.array.slice(0, stack.top + 1));

What is the output?

medium

A. [10, 20, 30, undefined, undefined]

B. [10, 20]

C. [10, 20, 30]

D. Error: Cannot read property 'slice' of undefined

Solution

1. Step 1: Trace push operations and resizing

   Initially array is empty. First push adds 10, array resizes to length 1 (from 0), then 10 stored at index 0. Second push resizes array from length 1 to 2, adds 20 at index 1. Third push resizes from 2 to 4, adds 30 at index 2.

2. Step 2: Slice array up to top index + 1

   top is 2 after three pushes, so slice(0, 3) returns [10, 20, 30]. Extra undefined elements are beyond slice range.

3. Final Answer:

   [10, 20, 30] -> Option C

4. Quick Check:

   Slice array to top+1 = [10, 20, 30] [OK]

Hint: Slice array up to top+1 to get valid stack elements [OK]

Common Mistakes:

• Expecting undefined elements in output
• Confusing array length with number of elements
• Not updating top index correctly

4. The following code attempts to implement pop operation for a dynamic stack but has a bug:

pop() {
  if (this.top === -1) {
    return null;
  }
  const value = this.array[this.top];
  this.array[this.top] = undefined;
  this.top--;
  if (this.top < this.array.length / 4) {
    this.array.length = Math.floor(this.array.length / 2);
  }
  return value;
}

What is the bug in this pop method?

medium

A. Reducing array length directly may remove elements still in use.

B. It does not check if stack is empty before popping.

C. It does not update the top index after popping.

D. It never shrinks the array size.

Solution

1. Step 1: Understand array shrinking logic

   The code shrinks the array length when top is less than quarter of array length.

2. Step 2: Identify problem with direct length reduction

   Reducing array length directly truncates elements beyond new length, which may include valid stack elements if top is not updated properly.

3. Final Answer:

   Reducing array length directly may remove elements still in use. -> Option A

4. Quick Check:

   Direct length cut risks losing valid elements [OK]

Hint: Avoid direct length cut; copy elements to smaller array instead [OK]

Common Mistakes:

• Ignoring empty stack check
• Not decrementing top index
• Assuming array never shrinks

5. You want to implement a dynamic stack that doubles its array size when full and halves it when the number of elements falls below one-fourth of the capacity. Which approach below correctly handles resizing without losing stack elements?

hard

A. Create a new array of the new size and copy existing elements before replacing the old array.

B. Directly set the array length property to new size to resize the array.

C. Use array.push() to add undefined elements to increase size and pop() to reduce size.

D. Do not resize the array; just keep track of top index.

Solution

1. Step 1: Understand resizing requirements

   Doubling and halving array size requires creating new arrays to avoid losing elements.

2. Step 2: Analyze resizing methods

   Create a new array of the new size and copy existing elements before replacing the old array. copies elements to a new array of correct size, preserving stack data safely.

3. Step 3: Why other options fail

   Directly set the array length property to new size to resize the array. truncates or extends array without copying, losing data. Use array.push() to add undefined elements to increase size and pop() to reduce size. misuses push/pop for resizing. Do not resize the array; just keep track of top index. ignores resizing.

4. Final Answer:

   Create a new array of the new size and copy existing elements before replacing the old array. -> Option A

5. Quick Check:

   Copy elements to new array for safe resizing [OK]

Hint: Always copy elements to new array when resizing [OK]

Common Mistakes:

• Setting length property directly to resize
• Using push/pop to resize array
• Ignoring resizing and risking overflow