Why Stack Exists and What Problems It Solves in DSA Typescript - Quick Recap

Recall & Review

beginner

What is a stack in simple terms?

A stack is like a pile of plates where you can only add or remove the top plate. It follows the Last In, First Out (LIFO) rule.

beginner

Why do we need a stack in programming?

Stacks help manage tasks that need to be done in reverse order, like undo actions, function calls, or checking matching brackets.

intermediate

How does a stack solve the problem of function calls in programming?

When functions call other functions, the stack keeps track of where to return after each call, ensuring the last called function finishes first.

intermediate

What problem does a stack solve in expression evaluation?

Stacks help evaluate expressions by storing operators and operands, especially when dealing with parentheses and operator precedence.

beginner

Explain how a stack helps in undo functionality.

Each action is pushed onto the stack. When undo is pressed, the last action is popped and reversed, allowing step-by-step undoing.

What order does a stack follow?

A Sorted Order

B Last In, First Out (LIFO)

C Random Order

D First In, First Out (FIFO)

A stack always removes the most recently added item first, which is Last In, First Out.

Which problem is best solved by using a stack?

A Tracking function calls

B Sorting numbers

C Searching in a list

D Storing unique items

Stacks keep track of function calls and return points, making them ideal for this problem.

What happens when you pop from an empty stack?

A You get an error or no value

B You get the first item added

C You get the last item added

D The stack resets

Popping from an empty stack usually causes an error or returns no value because there is nothing to remove.

How does a stack help with undo operations?

A By sorting actions alphabetically

B By storing only the first action

C By deleting all actions at once

D By storing actions in order and reversing the last one

Undo works by reversing the last action stored on the stack, following LIFO.

Which of these is NOT a typical use of a stack?

A Evaluating expressions

B Function call management

C Breadth-first search traversal

D Undo functionality

Breadth-first search uses a queue, not a stack.

Describe in your own words why a stack is useful in programming and give two examples of problems it solves.

Think about tasks that need to be reversed or tracked in order.

Explain how a stack manages function calls and why this is important for a program to run correctly.

Imagine stacking books and removing the top one first.

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