Why Stack Exists and What Problems It Solves in DSA Typescript - See It Work

Why Stack Exists and What Problems It Solves

📖 Scenario: Imagine you are organizing a stack of plates in a kitchen. You can only add or remove the top plate. This is how a stack data structure works in programming.Stacks help solve problems where the last thing added is the first thing you need to remove, like undo actions in apps or checking matching brackets in code.

🎯 Goal: You will create a simple stack using an array in TypeScript. You will add items, remove the top item, and see how the stack changes step-by-step.

📋 What You'll Learn

Create an empty stack as an array called stack

Add three items to the stack in order: 'plate1', 'plate2', 'plate3'

Remove the top item from the stack and store it in a variable called removedPlate

Print the current state of the stack after removal

💡 Why This Matters

🌍 Real World

Stacks are used in web browsers to keep track of pages you visit so you can go back in order.

💼 Career

Understanding stacks helps in coding interviews and building features like undo buttons or expression evaluators.

1

Create an empty stack

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 for the stack.

2

Add three plates to the stack

Add 'plate1', 'plate2', and 'plate3' to the stack array using the push method in this order.

DSA Typescript

const stack: string[] = [];
// Add 'plate1', 'plate2', and 'plate3' to stack using push
// Your code here

Hint

Use stack.push('plate1'); and similarly for the other plates to add them to the stack.

3

Remove the top plate from the stack

Remove the top item from stack using pop() and store it in a variable called removedPlate.

DSA Typescript

const stack: string[] = [];
stack.push('plate1');
stack.push('plate2');
stack.push('plate3');
// Remove the top plate and store in removedPlate
// Your code here

Hint

Use const removedPlate = stack.pop(); to remove the top plate and save it.

4

Print the current stack

Print the current state of stack using console.log(stack).

DSA Typescript

const stack: string[] = [];
stack.push('plate1');
stack.push('plate2');
stack.push('plate3');
const removedPlate = stack.pop();
// Print the current stack
// Your code here

Hint

Use console.log(stack); to see the stack after removing the top plate.

Practice

1. Why do we use a stack data structure in programming?

easy

A. To sort numbers in ascending order automatically

B. To store data in random order for quick access

C. To keep track of function calls and return in the correct order

D. To store large amounts of data permanently

Solution

1. Step 1: Understand the purpose of a stack

   A stack stores items in Last In First Out (LIFO) order, which is perfect for tracking function calls.

2. Step 2: Relate stack to function calls

   When a function calls another, the current function is paused and saved on the stack. When the called function finishes, the program returns to the last saved function.

3. Final Answer:

   To keep track of function calls and return in the correct order -> Option C

4. Quick Check:

   Stack = function call tracking [OK]

Hint: Stack follows LIFO, perfect for tracking nested calls [OK]

Common Mistakes:

• Thinking stack stores data randomly
• Confusing stack with queue
• Assuming stack sorts data automatically

2. Which of the following is the correct way to push an element '5' onto a stack named myStack in TypeScript?

easy

A. myStack.push(5);

B. myStack.add(5);

C. myStack.insert(5);

D. myStack.enqueue(5);

Solution

1. Step 1: Recall stack method names in TypeScript

   Stacks use push() to add elements on top.

2. Step 2: Identify correct method for adding element

   push() is the standard method; others like add(), insert(), or enqueue() are not used for stacks.

3. Final Answer:

   myStack.push(5); -> Option A

4. Quick Check:

   Push adds element on top [OK]

Hint: Use push() to add on stack top [OK]

Common Mistakes:

• Using enqueue which is for queues
• Using add or insert which are not stack methods
• Confusing push with pop

3. What will be the output of the following TypeScript code?

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

medium

A. [30]

B. [10, 20, 30]

C. [20, 30]

D. [10, 30]

Solution

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

   Initially stack is empty []. Push 10 -> [10]. Push 20 -> [10, 20]. Pop removes 20 -> [10]. Push 30 -> [10, 30].

2. Step 2: Determine final stack content

   After all operations, stack contains [10, 30].

3. Final Answer:

   [10, 30] -> Option D

4. Quick Check:

   Push/pop changes stack top [OK]

Hint: Pop removes last pushed element [OK]

Common Mistakes:

• Forgetting pop removes top element
• Assuming pop returns stack without removing
• Mixing order of push and pop

4. Find the error in this TypeScript stack usage code:

const stack: number[] = [];
stack.push(1);
stack.push(2);
const top = stack.peek();
console.log(top);

medium

A. Stack does not have a peek() method by default

B. push() method is used incorrectly

C. stack should be declared as Stack<number> not number[]

D. console.log cannot print stack elements

Solution

1. Step 1: Check methods available on array used as stack

   TypeScript arrays have push() and pop(), but no peek() method by default.

2. Step 2: Identify cause of error

   Calling stack.peek() will cause a runtime error because peek() is not defined.

3. Final Answer:

   Stack does not have a peek() method by default -> Option A

4. Quick Check:

   peek() missing in array stack [OK]

Hint: Array stack has pop(), no peek() [OK]

Common Mistakes:

• Assuming peek() exists on arrays
• Thinking push() is wrong
• Confusing type declaration with method error

5. You want to reverse a string using a stack in TypeScript. Which approach correctly uses a stack to reverse the string "hello"?

hard

A. Push characters and pop in same order to keep string unchanged

B. Push each character onto stack, then pop all to build reversed string

C. Use stack to sort characters alphabetically

D. Push entire string onto stack, then pop once to get reversed string

Solution

1. Step 1: Understand stack reversal logic

   Pushing characters one by one stores them in order. Popping them reverses the order because stack is LIFO.

2. Step 2: Apply to string reversal

   Push 'h', 'e', 'l', 'l', 'o' then pop all to get 'o', 'l', 'l', 'e', 'h' which is reversed string.

3. Final Answer:

   Push each character onto stack, then pop all to build reversed string -> Option B

4. Quick Check:

   Stack LIFO reverses order [OK]

Hint: Push chars, pop all to reverse string [OK]

Common Mistakes:

• Pushing entire string at once
• Popping in same order as push
• Using stack for sorting instead of reversing