Stack Implementation Using Array in DSA Typescript - Build from Scratch

Stack Implementation Using Array

📖 Scenario: Imagine you are managing a stack of books on a table. You can only add or remove the top book. This is how a stack data structure works.

🎯 Goal: You will build a simple stack using an array in TypeScript. You will add items, remove the top item, and then display the current stack.

📋 What You'll Learn

Create an empty array called stack to represent the stack.

Create a variable called maxSize and set it to 5 to limit the stack size.

Add three items to the stack array using the push method.

Remove the top item from the stack array using the pop method.

Print the current state of the stack array.

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world applications like undo features in text editors, browser history, and managing function calls.

💼 Career

Understanding stack implementation helps in software development roles where managing data in a last-in-first-out order is required.

1

Create an empty stack array

Create an empty array called stack to represent the stack.

DSA Typescript

// Create an empty array called stack
// Your code here

Hint

Use const stack: string[] = []; to create an empty array of strings.

2

Set the maximum stack size

Create a variable called maxSize and set it to 5 to limit the stack size.

DSA Typescript

const stack: string[] = [];
// Create a variable maxSize and set it to 5
// Your code here

Hint

Use const maxSize: number = 5; to set the stack size limit.

3

Add three items to the stack

Add these three items to the stack array using the push method in this order: 'Book1', 'Book2', 'Book3'.

DSA Typescript

const stack: string[] = [];
const maxSize: number = 5;
// Add 'Book1', 'Book2', and 'Book3' to stack using push
// Your code here

Hint

Use stack.push('Book1'); to add items to the stack.

4

Remove the top item and print the stack

Remove the top item from the stack array using the pop method. Then print the current state of the stack array using console.log(stack).

DSA Typescript

const stack: string[] = [];
const maxSize: number = 5;
stack.push('Book1');
stack.push('Book2');
stack.push('Book3');
// Remove the top item using pop
// Print the stack
// Your code here

Hint

Use stack.pop(); to remove the top item and console.log(stack); to print the stack.

Practice

1. What is the main characteristic of a stack data structure implemented using an array?

easy

A. It stores elements in sorted order

B. It follows First In First Out (FIFO) order

C. It allows random access to elements

D. It follows Last In First Out (LIFO) order

Solution

1. Step 1: Understand stack behavior

   A stack adds and removes elements from the same end, so the last element added is the first to be removed.

2. Step 2: Identify the order type

   This behavior is called Last In First Out (LIFO).

3. Final Answer:

   It follows Last In First Out (LIFO) order -> Option D

4. Quick Check:

   Stack = LIFO [OK]

Hint: Stack always removes the last added item first [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking stack sorts elements
• Assuming random access like arrays

2. Which of the following is the correct way to push an element 'x' onto a stack implemented as an array named stack in TypeScript?

easy

A. stack.insert(x);

B. stack.pop(x);

C. stack.push(x);

D. stack.add(x);

Solution

1. Step 1: Recall array methods for stack push

   In TypeScript, to add an element at the end of an array, use the push method.

2. Step 2: Verify other options

   pop removes elements, insert and add are not valid array methods.

3. Final Answer:

   stack.push(x); -> Option C

4. Quick Check:

   Push uses .push() method [OK]

Hint: Use .push() to add elements to array stack [OK]

Common Mistakes:

• Using pop instead of push
• Using non-existent methods like insert or add
• Confusing push with pop

3. What will be the output of the following TypeScript code implementing a stack using an array?

const stack: number[] = [];
stack.push(10);
stack.push(20);
stack.pop();
stack.push(30);
console.log(stack);

medium

A. [10, 20, 30]

B. [10, 30]

C. [20, 30]

D. [30]

Solution

1. Step 1: Trace stack operations step-by-step

   Initially stack = []. After push(10), stack = [10]. After push(20), stack = [10, 20]. After pop(), top element 20 is removed, stack = [10]. After push(30), stack = [10, 30].

2. Step 2: Determine final stack state

   The console.log prints the current stack array: [10, 30].

3. Final Answer:

   [10, 30] -> Option B

4. Quick Check:

   Push and pop update stack correctly [OK]

Hint: Pop removes last pushed element, then push adds new [OK]

Common Mistakes:

• Forgetting pop removes last element
• Assuming pop removes first element
• Confusing order of push and pop

4. Identify the error in the following TypeScript code for stack pop operation and select the correct fix.

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

medium

A. pop() should check if stack is empty before popping

B. items array should be public, not private

C. pop() should return void, not number

D. No error, code is correct

Solution

1. Step 1: Analyze pop method behavior

   The pop() method of array returns number | undefined if empty. Returning directly without check can cause undefined return.

2. Step 2: Identify fix for empty stack

   To avoid returning undefined, pop() should check if items is empty before popping and handle that case.

3. Final Answer:

   pop() should check if stack is empty before popping -> Option A

4. Quick Check:

   Check empty before pop to avoid undefined [OK]

Hint: Always check stack empty before pop to avoid errors [OK]

Common Mistakes:

• Ignoring empty stack check
• Changing return type incorrectly
• Making items public unnecessarily

5. Given a stack implemented using an array, how would you implement a function reverseStack that reverses the elements in the stack without using any extra stack or array? Which approach is correct?

hard

A. Use recursion to pop all elements, then insert them at bottom recursively

B. Use a for loop to swap elements from start and end indices

C. Pop all elements into a new array, then push back in reverse order

D. Use a queue to dequeue and enqueue elements to reverse order

Solution

1. Step 1: Understand constraints

   No extra stack or array allowed, so iterative swapping or extra storage is not possible.

2. Step 2: Use recursion to reverse stack

   Recursion can pop all elements, then insert each element at the bottom of the stack recursively, effectively reversing it.

3. Step 3: Evaluate other options

   Use a for loop to swap elements from start and end indices requires direct index access which is not typical for stack abstraction. Pop all elements into a new array, then push back in reverse order uses extra array. Use a queue to dequeue and enqueue elements to reverse order uses queue, which is extra data structure.

4. Final Answer:

   Use recursion to pop all elements, then insert them at bottom recursively -> Option A

5. Quick Check:

   Recursion reverses stack without extra storage [OK]

Hint: Recursion can reverse stack by inserting at bottom [OK]

Common Mistakes:

• Using extra arrays or stacks against constraints
• Trying to swap elements directly in stack
• Using other data structures like queue