Stack Concept and LIFO Principle in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this stack operation sequence?

Consider a stack initially empty. We perform these operations in order:
push(10), push(20), pop(), push(30), pop(), push(40), push(50), pop()
What is the content of the stack from top to bottom after these operations?

DSA Typescript

class Stack {
  private items: number[] = [];
  push(item: number) { this.items.push(item); }
  pop() { return this.items.pop(); }
  print() { return this.items.slice().reverse().join(' -> ') + ' -> null'; }
}

const stack = new Stack();
stack.push(10);
stack.push(20);
stack.pop();
stack.push(30);
stack.pop();
stack.push(40);
stack.push(50);
stack.pop();
console.log(stack.print());

A 50 -> 40 -> 10 -> null

B 40 -> 10 -> null

C 40 -> 30 -> 10 -> null

D 30 -> 10 -> null

💡 Hint

Remember pop removes the top item, and push adds on top.

After all operations, the stack has 10 at bottom and 40 on top. The last pop removed 50.

🧠 Conceptual

intermediate

Which property best describes a stack?

A stack is a data structure that follows which principle?

A Sorted Order

B First In, First Out (FIFO)

C Random Access

D Last In, First Out (LIFO)

💡 Hint

Think about which element is removed first.

Stacks remove the most recently added element first, so they follow Last In, First Out (LIFO).

🔧 Debug

advanced

What error occurs when popping from an empty stack?

Given this TypeScript stack class, what happens if pop() is called on an empty stack?

DSA Typescript

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

const stack = new Stack();
stack.pop();

A Returns undefined without error

B Throws RangeError

C Throws TypeError

D Throws ReferenceError

💡 Hint

Check what Array.pop() returns when empty.

Array.pop() returns undefined if the array is empty; no error is thrown.

🚀 Application

advanced

How to reverse a string using a stack?

Which code snippet correctly reverses the string 'hello' using a stack?

A Pop chars first, then push to reverse

B Push all chars, then pop only half to reverse

C Push each char, then pop all to build reversed string

D Use stack to sort chars alphabetically

💡 Hint

Think about pushing all characters first, then popping to reverse order.

Pushing all characters then popping them reverses the string due to LIFO.

❓ Predict Output

expert

What is the final stack content after this complex sequence?

Given this sequence of stack operations, what is the final stack content from top to bottom?
push(1), push(2), push(3), pop(), push(4), pop(), pop(), push(5), push(6), pop()

DSA Typescript

class Stack {
  private items: number[] = [];
  push(item: number) { this.items.push(item); }
  pop() { return this.items.pop(); }
  print() { return this.items.slice().reverse().join(' -> ') + ' -> null'; }
}

const stack = new Stack();
stack.push(1);
stack.push(2);
stack.push(3);
stack.pop();
stack.push(4);
stack.pop();
stack.pop();
stack.push(5);
stack.push(6);
stack.pop();
console.log(stack.print());

A 5 -> 1 -> null

B 6 -> 5 -> 1 -> null

C 2 -> 1 -> null

D 1 -> null

💡 Hint

Trace each push and pop carefully to see what remains.

After all operations, stack has 1 at bottom and 5 on top.

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