Stack Concept and LIFO Principle in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack in data structures?

A stack is a collection where elements are added and removed only from the top. It follows the Last In, First Out (LIFO) principle.

beginner

Explain the LIFO principle in simple terms.

LIFO means the last item you put in is the first one you take out, like a stack of plates where you take the top plate first.

beginner

What are the two main operations of a stack?

The two main operations are push (to add an item on top) and pop (to remove the top item).

beginner

If you push elements 1, 2, 3 in order, what will be the order when popping all elements?

The elements will come out in reverse order: 3, then 2, then 1, because of the LIFO principle.

intermediate

Why is a stack useful in programming?

Stacks help manage tasks like undo actions, function calls, and expression evaluation by keeping track of the order of operations.

Which operation adds an element to the top of a stack?

A Insert

B Push

C Peek

D Pop

Push adds an element to the top of the stack.

What does LIFO stand for?

A Last In, First Out

B Last In, Final Out

C List In, First Out

D Last Input, First Output

LIFO means Last In, First Out.

If a stack has elements [1, 2, 3] with 3 on top, which element will pop out first?

A 1

B 2

C 3

D None

The top element 3 will pop out first.

Which of these is NOT a typical stack operation?

A Push

B Pop

C Peek

D Sort

Sort is not a standard stack operation.

What real-life example best illustrates a stack?

A A pile of books where you take the top book first

B A queue at a ticket counter

C A list of names in alphabetical order

D A random set of cards

A pile of books where you take the top book first is like a stack.

Describe how the LIFO principle works in a stack with an example.

Think about a stack of plates or books.

List and explain the main operations of a stack and their effects.

Focus on what happens at the top 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