Stack Concept and LIFO Principle in DSA Java - Execution Trace

Concept Flow - Stack Concept and LIFO Principle

Start with empty stack

↓

Push element on top

→Top points to new element

↓

Push more elements?

Yes→Repeat push

No↓

Pop element from top

→Remove top element

↓

Pop more elements?

Yes→Repeat pop

No↓

Stack empty or done

Stack adds elements on top and removes from top, following Last In First Out (LIFO) order.

Execution Sample

DSA Java

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

Push 10, 20, 30 onto stack, then pop twice, showing LIFO order.

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Initialize empty stack	[]	top = null	null
2	push(10)	[10]	top -> 10	10 -> null
3	push(20)	[20, 10]	top -> 20	20 -> 10 -> null
4	push(30)	[30, 20, 10]	top -> 30	30 -> 20 -> 10 -> null
5	pop()	[20, 10]	top -> 20	20 -> 10 -> null
6	pop()	[10]	top -> 10	10 -> null
7	End	[10]	top -> 10	10 -> null

💡 No more operations; stack has one element left.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
stack size	0	1	2	3	2	1	1
top element	null	10	20	30	20	10	10

Key Moments - 3 Insights

Why does pop remove the last pushed element, not the first?

Because stack follows LIFO: last element pushed is on top and removed first, as shown in steps 4 to 6 in execution_table.

What happens if we pop when stack is empty?

Pop would cause error or return null because top is null (empty stack), but this example stops before empty to avoid error.

Why does push add element on top, not bottom?

Push adds on top so the newest element is always removed first, maintaining LIFO, as seen in steps 2 to 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the top element after step 3?

A 10

B 20

C 30

D null

💡 Hint

Check 'Pointer Changes' and 'Visual State' columns at step 3 (push 20).

At which step does the stack size reduce from 3 to 2?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look at 'stack size' in variable_tracker between steps 4 and 5.

If we add push(40) after step 4, what would be the new top element?

A 30

B 20

C 40

D 10

💡 Hint

Push adds element on top, so new element becomes top.

Concept Snapshot

Stack is a data structure that stores elements in LIFO order.
Use push() to add on top, pop() to remove from top.
Top always points to the last pushed element.
Pop removes the most recent element first.
Stack size changes with each push/pop.
Empty stack has top = null.

Full Transcript

A stack stores items so the last one added is the first one removed, called Last In First Out or LIFO. We start with an empty stack. When we push 10, it becomes the top. Then pushing 20 puts it on top, pushing 30 puts it on top again. When we pop, we remove the top element, so popping removes 30 first, then 20. The stack size changes accordingly. The top pointer always points to the newest element. This example shows how push and pop work step-by-step, with the stack's visual state after each operation.

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