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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this stack operation sequence?

Consider a stack where we push integers 1, 2, 3 in order, then pop one element, then push 4. What is the printed stack from bottom to top?

DSA Java

Stack<Integer> stack = new Stack<>();
stack.push(1);
stack.push(2);
stack.push(3);
stack.pop();
stack.push(4);
System.out.print(stack);

A [1, 2, 3, 4]

B [4, 2, 1]

C [1, 2, 4]

D [2, 4]

💡 Hint

Remember stack pops the last pushed item first.

We push 1, 2, 3. Stack is [1, 2, 3]. Pop removes 3. Stack is [1, 2]. Push 4 adds on top. Stack is [1, 2, 4].

🧠 Conceptual

intermediate

Which operation follows the LIFO principle in a stack?

Choose the operation that always removes the most recently added element from a stack.

A pop

B peek

C push

D isEmpty

💡 Hint

LIFO means last in, first out.

Pop removes the last element added, following LIFO. Push adds, peek views top, isEmpty checks emptiness.

🔧 Debug

advanced

What error occurs when popping from an empty stack?

Given this code snippet, what error will it throw when pop() is called on an empty stack?

DSA Java

Stack<Integer> stack = new Stack<>();
stack.pop();

A NoSuchElementException

B EmptyStackException

C IndexOutOfBoundsException

D NullPointerException

💡 Hint

Check Java stack exception types for empty pop.

Java's Stack class throws EmptyStackException when pop is called on empty stack.

🚀 Application

advanced

What is the stack content after this sequence of operations?

Start with an empty stack. Perform: push(5), push(10), pop(), push(15), pop(), push(20). What is the stack from bottom to top?

DSA Java

Stack<Integer> stack = new Stack<>();
stack.push(5);
stack.push(10);
stack.pop();
stack.push(15);
stack.pop();
stack.push(20);
System.out.print(stack);

A [10, 20]

B [20, 5]

C [5, 15, 20]

D [5, 20]

💡 Hint

Pop removes the last pushed element each time.

Push 5: [5]
Push 10: [5,10]
Pop removes 10: [5]
Push 15: [5,15]
Pop removes 15: [5]
Push 20: [5,20]

❓ Predict Output

expert

What is the output of this Java stack code with multiple pops?

Given the code below, what is printed after all operations?

DSA Java

Stack<String> stack = new Stack<>();
stack.push("a");
stack.push("b");
stack.push("c");
stack.pop();
stack.pop();
stack.push("d");
System.out.print(stack);

A [a, d]

B [d, a]

C [a, b, d]

D [a, b]

💡 Hint

Pop removes last pushed elements in order.

Push a: [a]
Push b: [a,b]
Push c: [a,b,c]
Pop removes c: [a,b]
Pop removes b: [a]
Push d: [a,d]

Practice

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

easy

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

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

C. Elements are removed randomly

D. Elements are removed based on their value

Solution

1. Step 1: Understand the LIFO principle

   LIFO means the last item you put in is the first one you take out.

2. Step 2: Apply to stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

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

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack like a stack of plates, last on top removed first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking elements are removed randomly
• Assuming removal depends on element value

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

easy

A. stack.insert(5);

B. stack.add(5);

C. stack.put(5);

D. stack.push(5);

Solution

1. Step 1: Recall Java Stack method for adding elements

   The method to add an element to a stack is push().

2. Step 2: Match method with given options

   Only stack.push(5); correctly uses the stack push method.

3. Final Answer:

   stack.push(5); -> Option D

4. Quick Check:

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

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

Common Mistakes:

• Using add() which is for lists, not stacks
• Using insert() or put() which are invalid for Stack
• Confusing stack methods with other collections

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

Stack<Integer> stack = new Stack<>();
stack.push(10);
stack.push(20);
stack.push(30);
System.out.println(stack.pop());
System.out.println(stack.pop());

medium

A. 30\n20

B. 30\n10

C. 20\n30

D. 10\n20

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 10 (bottom), 20, 30 (top).

2. Step 2: Trace pop operations

   First pop removes 30, second pop removes 20.

3. Final Answer:

   30\n20 -> Option A

4. Quick Check:

   Stack pops last pushed first: 30 then 20 [OK]

Hint: Pop returns last pushed element first [OK]

Common Mistakes:

• Assuming pop removes from bottom
• Mixing order of popped elements
• Confusing push and pop effects

4. Identify the error in the following Java code snippet using a stack:

Stack<String> stack = new Stack<>();
stack.push("apple");
stack.push("banana");
String fruit = stack.pop();
System.out.println(stack.peek());
stack.pop();
stack.pop();

medium

A. Using peek() instead of pop() is incorrect

B. Stack cannot hold String objects

C. Calling pop() on an empty stack causes EmptyStackException

D. push() method is not defined for Stack

Solution

1. Step 1: Trace stack operations

   Push "apple", then "banana". Pop removes "banana". Peek shows "apple". Next pop removes "apple".

2. Step 2: Identify error on last pop

   Last pop tries to remove from empty stack, causing EmptyStackException.

3. Final Answer:

   Calling pop() on an empty stack causes EmptyStackException -> Option C

4. Quick Check:

   Pop on empty stack throws exception [OK]

Hint: Pop more times than pushes causes error [OK]

Common Mistakes:

• Ignoring exception on empty pop
• Thinking peek removes element
• Believing stack can't hold Strings

5. Given a stack with elements pushed in this order: 1, 2, 3, 4, 5. Which sequence of pop operations will produce the output 5 4 3 and leave the stack with elements 1 2 remaining?

hard

A. Pop three times consecutively

B. Pop once, push 6, then pop twice

C. Pop twice, push 7, then pop once

D. Pop once, pop twice, push 8

Solution

1. Step 1: Understand initial stack order

   Stack top is 5, bottom is 1 after pushing 1 to 5.

2. Step 2: Pop three times consecutively

   Pop removes 5, then 4, then 3, leaving 1 and 2 in stack.

3. Final Answer:

   Pop three times consecutively -> Option A

4. Quick Check:

   3 pops remove top 3 elements [OK]

Hint: Pop as many times as output elements needed [OK]

Common Mistakes:

• Adding elements unnecessarily
• Confusing push and pop order
• Expecting output without popping