Dynamic Stack Using Resizable Array in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a dynamic stack using a resizable array?

A dynamic stack is a stack data structure that can grow or shrink in size automatically by resizing its underlying array when needed, allowing flexible storage without fixed limits.

intermediate

How does a dynamic stack handle overflow when pushing elements?

When the stack's array is full, the dynamic stack creates a new larger array (usually double the size), copies all elements to it, and then adds the new element, allowing more space.

intermediate

What is the time complexity of push operation in a dynamic stack using a resizable array?

Most push operations take O(1) time, but occasionally when resizing is needed, it takes O(n) time to copy elements. Amortized over many pushes, the average time is still O(1).

intermediate

Why do we shrink the array size in a dynamic stack?

Shrinking the array when many elements are popped saves memory by reducing unused space, keeping the stack size efficient and preventing waste.

beginner

Show the basic steps of the pop operation in a dynamic stack using a resizable array.

1. Check if stack is empty; if yes, return error.
2. Remove and return the top element.
3. Decrease the size count.
4. If size is much smaller than array capacity, shrink the array.

What happens when you push an element onto a full dynamic stack array?

A The array size doubles and elements are copied over

B The push operation fails with overflow error

C The oldest element is removed to make space

D The array size halves automatically

When the array is full, the dynamic stack doubles its size and copies existing elements to the new array before adding the new element.

What is the average time complexity of push in a dynamic stack using a resizable array?

A O(n)

B O(1)

C O(log n)

D O(n^2)

Although resizing takes O(n), it happens rarely, so the average (amortized) time for push is O(1).

Why might a dynamic stack shrink its array size?

A To sort the elements

B To increase push speed

C To avoid stack underflow

D To save memory when many elements are popped

Shrinking reduces unused memory when the stack has fewer elements, making memory use efficient.

Which operation is NOT typical in a dynamic stack using a resizable array?

A Random access by index

B Pop

C Resize

D Push

Stacks do not support random access; elements are accessed only from the top.

What is the first step when popping an element from a dynamic stack?

A Resize the array

B Double the array size

C Check if the stack is empty

D Add a new element

Before popping, we check if the stack is empty to avoid errors.

Explain how a dynamic stack using a resizable array manages its size during push and pop operations.

Think about what happens when the stack is full or mostly empty.

Describe the advantages of using a dynamic stack with a resizable array compared to a fixed-size stack.

Consider flexibility and performance.

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