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DSA Javaprogramming

Stack Concept and LIFO Principle in DSA Java

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Mental Model
A stack is a collection where the last item added is the first one to be removed.
Analogy: Think of a stack of plates: you put new plates on top and take plates from the top only.
Top
 ↑
[ 3 ]
[ 2 ]
[ 1 ]
Bottom
Dry Run Walkthrough
Input: Start with empty stack, push 1, push 2, push 3, then pop one item
Goal: Show how items are added and removed following last-in-first-out order
Step 1: Push 1 onto the stack
Top
 ↑
[ 1 ]
Bottom
Why: We add the first item at the top of the stack
Step 2: Push 2 onto the stack
Top
 ↑
[ 2 ]
[ 1 ]
Bottom
Why: New item goes on top, above the previous one
Step 3: Push 3 onto the stack
Top
 ↑
[ 3 ]
[ 2 ]
[ 1 ]
Bottom
Why: Keep adding items on top, last pushed is on top
Step 4: Pop the top item from the stack
Top
 ↑
[ 2 ]
[ 1 ]
Bottom
Why: Remove the last item added, which is on top
Result: 
Top
 ↑
[ 2 ]
[ 1 ]
Bottom
Popped item: 3
Annotated Code
DSA Java
import java.util.Stack;

public class StackDemo {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<>();

        // Push 1
        stack.push(1); // add 1 on top

        // Push 2
        stack.push(2); // add 2 on top

        // Push 3
        stack.push(3); // add 3 on top

        // Pop top item
        int popped = stack.pop(); // remove top item (3)

        // Print stack content
        System.out.println("Stack from top to bottom:");
        for (int i = stack.size() - 1; i >= 0; i--) {
            System.out.println(stack.get(i));
        }

        // Print popped item
        System.out.println("Popped item: " + popped);
    }
}
stack.push(1); // add 1 on top
push first item onto empty stack
stack.push(2); // add 2 on top
push second item on top of stack
stack.push(3); // add 3 on top
push third item on top of stack
int popped = stack.pop(); // remove top item (3)
pop removes last pushed item from top
for (int i = stack.size() - 1; i >= 0; i--) {
iterate from top to bottom to print stack
OutputSuccess
Stack from top to bottom: 3 2 1 Popped item: 3
Complexity Analysis
Time: O(1) for push and pop because these operations only add or remove the top item without traversing the stack
Space: O(n) because the stack stores n items in memory
vs Alternative: Compared to arrays where adding/removing at the end can be O(1), stack operations are similarly efficient but enforce strict last-in-first-out order
Edge Cases
Pop from empty stack
Throws EmptyStackException to signal no items to remove
DSA Java
int popped = stack.pop(); // remove top item (3)
When to Use This Pattern
When you see a problem requiring last item added to be processed first, reach for the stack pattern because it naturally follows last-in-first-out order.
Common Mistakes
Mistake: Trying to remove items from the bottom or middle of the stack
Fix: Always remove only from the top using pop to maintain LIFO order
Mistake: Confusing stack order with queue order (FIFO)
Fix: Remember stack removes last added item first, unlike queue which removes first added item first
Summary
A stack stores items so the last one added is the first one removed.
Use a stack when you need to reverse order or track recent items first.
The key is last-in-first-out: only the top item is accessible for removal.

Practice

(1/5)
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
Recommended

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10 questions~5 mineasy hard