feat(ArrayListDemo5): show ArrayList used as a simple stack (peek, pop)

What
- Added ArrayListDemo5 class.
- Demonstrated how an ArrayList can be used like a stack:
  - push → add(E).
  - peek → get(size-1).
  - pop → remove(size-1).
- Inserted elements [1,2,3].
- Called get(lastIndex) to peek at top element.
- Called remove(lastIndex) to pop top element.

Why
- Illustrates that ArrayList can mimic stack behavior with add/get/remove at end.
- Helps understand relationship between dynamic arrays and stack operations.
- Shows why this is not recommended for production when java.util.Stack or Deque exists.

How
- ArrayLists.add(1,2,3) → list = [1,2,3].
- Peek:
  - ArrayLists.get(2) → returns 3 (top element).
- Pop:
  - ArrayLists.remove(2) → removes 3, list becomes [1,2].

Logic
- Inputs: integers pushed via add().
- Outputs: peek and pop values (not printed here, but executed).
- Flow:
  1. Add elements sequentially.
  2. Peek last element with get(size-1).
  3. Remove last element with remove(size-1).
- Edge cases:
  - Calling get(size-1) or remove(size-1) on empty list throws IndexOutOfBoundsException.
- Complexity:
  - add(E) amortized O(1).
  - get(size-1), remove(size-1) O(1).
- Concurrency: ArrayList not thread-safe.

Real-life applications
- Temporary stack-like use when only last-in/first-out needed and no Stack/Deque required.
- Quick demonstrations of LIFO without importing extra classes.

Notes
- Not recommended as best practice:
  - ArrayList lacks explicit push, pop, peek methods.
  - java.util.Stack or ArrayDeque is the idiomatic choice in Java.
- This demo is educational to show structural similarity, not intended as robust design.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section25CollectionFramework/src/ListDemo/ArrayListLinkedListStack/UseArrayListAsStack.java renamed to Section 25 Collections Frameworks/List Interface/ArrayList/src/ArrayListDemo5.java

@@ -1,9 +1,8 @@
-package ListDemo.ArrayListLinkedListStack;
-
 import java.util.ArrayList;
 
-public class UseArrayListAsStack {
+public class ArrayListDemo5 {
     public static void main(String[] args) {
+        // Use Arraylist as a Stack.
         ArrayList<Integer> ArrayLists = new ArrayList<>();
         ArrayLists.add(1);
         ArrayLists.add(2);
@@ -13,7 +12,5 @@ public static void main(String[] args) {
         ArrayLists.remove(ArrayLists.size()-1); //Pop
     }
 }
-/*
-arraylist as stack?
-Not good practice...In built methods are not available.
-*/
+
+// arraylist as stack? Not good practice...In built methods they are not available.