Stack Concept and LIFO Principle in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA Typescript

stack.[1](5);

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

Apop

Bpush

Cpeek

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push

Using peek which only views the top element

The push method adds an element to the top of the stack, following the LIFO principle.

2fill in blank

medium

Complete the code to remove the top element from the stack.

DSA Typescript

const top = stack.[1]();

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

Aunshift

Bpush

Cshift

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop

Using shift which removes from the front

The pop method removes and returns the top element of the stack, following LIFO.

3fill in blank

hard

Complete the code to check the top element without removing it.

DSA Typescript

const top = stack[stack.[1] - 1];

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

Apush

Bpop

Clength

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes the element

Using size which does not exist on arrays

To check the top element without removing it, access stack[stack.length - 1].

4fill in blank

hard

Fill both blanks to create a stack and add an element to it.

DSA Typescript

const stack = new [1]();
stack.[2](10);

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

AArray

Bpush

Cpop

DSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using Set instead of Array

Using pop to add elements

We create a stack using an Array and add an element using push.

5fill in blank

hard

Fill all three blanks to remove the top element and check the stack size.

DSA Typescript

const removed = stack.[1]();
const size = stack.[2];
console.log('Removed:', removed, 'Size:', [3]);

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

Apop

Blength

Csize

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop

Using size property which does not exist on arrays

pop() removes the top element, length gives the current size of the stack.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the last element added is the first to be removed

B. Last In, First Out - the first element added is the first to be removed

C. First In, First Out - the first element added is the first to be removed

D. First In, Last Out - the first element added is the last to be removed

Solution

1. Step 1: Understand LIFO meaning

   LIFO means the last element added to the stack is the first one to be removed.

2. Step 2: Match with options

   Last In, First Out - the last element added is the first to be removed correctly describes this behavior as "Last In, First Out".

3. Final Answer:

   Last In, First Out - the last element added is the first to be removed -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing up the order of insertion and removal

2. Which of the following is the correct TypeScript syntax to push an element `5` onto a stack array named stack?

easy

A. stack.push(5);

B. stack.add(5);

C. stack.insert(5);

D. stack.append(5);

Solution

1. Step 1: Recall array method to add element at end

   In TypeScript, the method to add an element at the end of an array is push().

2. Step 2: Verify options

   Only stack.push(5); uses push(5), which is correct syntax to add 5 to the stack.

3. Final Answer:

   stack.push(5); -> Option A

4. Quick Check:

   Use push() to add element to stack [OK]

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

Common Mistakes:

• Using non-existent methods like add or insert
• Confusing push with append (not in JS/TS)
• Syntax errors like missing parentheses

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

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

medium

A. [10, 20, 30]

B. [20, 30]

C. [10, 20]

D. [30]

Solution

1. Step 1: Trace push operations

   Elements 10, 20, and 30 are pushed in order, so stack becomes [10, 20, 30].

2. Step 2: Trace pop operation

   Pop removes the last element (30), so stack becomes [10, 20].

3. Final Answer:

   [10, 20] -> Option C

4. Quick Check:

   Pop removes last item, stack left with [10, 20] [OK]

Hint: Pop removes last pushed element [OK]

Common Mistakes:

• Thinking pop removes first element
• Confusing push and pop effects
• Expecting stack unchanged after pop

4. Identify the error in the following TypeScript stack code:

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

medium

A. console.log syntax is wrong

B. stack array is not initialized

C. push method is used incorrectly

D. pop method is not called with parentheses

Solution

1. Step 1: Check pop usage

   The code assigns stack.pop without parentheses, so it assigns the function itself, not the popped value.

2. Step 2: Correct usage

   To pop and get the value, it should be stack.pop() with parentheses to call the method.

3. Final Answer:

   pop method is not called with parentheses -> Option D

4. Quick Check:

   Call pop() with () to execute [OK]

Hint: Always use parentheses to call pop() [OK]

Common Mistakes:

• Forgetting parentheses on pop()
• Confusing method reference with method call
• Assuming pop returns function instead of value

5. Given a stack implemented as an array, which sequence of operations will result in the stack containing [1, 2, 3] after all operations?

Choose the correct sequence:
A) push(1), push(2), pop(), push(3), push(2)
B) push(1), pop(), push(2), push(3), push(1)
C) push(3), push(2), push(1), pop(), push(1)
D) push(1), push(2), push(3), pop(), push(3)

hard

A. push(1), push(2), pop(), push(3), push(2)

B. push(1), push(2), push(3), pop(), push(3)

C. push(3), push(2), push(1), pop(), push(1)

D. push(1), pop(), push(2), push(3), push(1)

Solution

1. Step 1: Trace push(1), push(2), push(3), pop(), push(3) operations

   Start with empty stack []:
   push(1) -> [1]
   push(2) -> [1, 2]
   push(3) -> [1, 2, 3]
   pop() removes 3 -> [1, 2]
   push(3) -> [1, 2, 3]

2. Step 2: Verify final stack

   After these operations, stack is [1, 2, 3], matching the target.

3. Final Answer:

   push(1), push(2), push(3), pop(), push(3) -> Option B

4. Quick Check:

   Pop removes last, push adds correct element [OK]

Hint: Trace each push/pop step carefully [OK]

Common Mistakes:

• Not tracking pop effect correctly
• Assuming pop removes first element
• Mixing order of pushes and pops