feat(ListIteratorDemo): add demo of bidirectional traversal with ListIterator

What
- Added ListIteratorDemo class.
- Created a List<Integer> with values [5, 10, 50, 60, 70, 80, 90].
- Obtained ListIterator<Integer> from list.listIterator().
- Demonstrated:
  - Forward traversal using hasNext() / next().
  - Printed each value along with hasPrevious() status.
  - Backward traversal using hasPrevious() / previous().
  - Printed each value when moving backward.

Why
- Shows how ListIterator extends Iterator with bidirectional navigation.
- Demonstrates practical use of hasPrevious() / previous() in addition to forward traversal.
- Highlights that ListIterator can both move forward and then move back through the same list.

How
- list.listIterator() starts cursor before first element.
- Forward traversal:
  - While hasNext() → consume next().
  - For each element, print next() result and current hasPrevious() state.
- Backward traversal:
  - Once cursor is at end (after forward traversal), hasPrevious() becomes true.
  - While hasPrevious() → consume previous().
  - Prints values in reverse order.

Logic
- Inputs: list [5, 10, 50, 60, 70, 80, 90].
- Outputs:
  - Forward traversal prints numbers in order (5→90).
  - Backward traversal prints numbers in reverse (90→5).
- Flow:
  1. Forward loop consumes all elements.
  2. Iterator ends at list.size().
  3. Backward loop consumes elements in reverse until start.
- Edge cases:
  - At start, hasPrevious() false.
  - At end, hasNext() false.
  - Calling next() without hasNext() or previous() without hasPrevious() throws NoSuchElementException.
- Complexity / performance: O(n) traversal forward + backward.
- Concurrency / thread-safety: Fail-fast under structural modification; not thread-safe.
- Error handling: Not needed in demo.

Real-life applications
- Implementing editors, undo/redo features where bidirectional movement is required.
- Navigating playlists, browsing history, or doubly linked data.
- Performing modifications in-place with set(), add(), or remove() during iteration.

Notes
- ListIterator also provides methods to modify the list during traversal.
- More powerful than plain Iterator, but specific to List collections.
- Cursor position moves between elements; after forward traversal, it's positioned after last element.

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

Author: Someshdiwan

Section 25 Collections Frameworks/List Interface/ArrayList/Iterator/src/ListIteratorDemo.java

@@ -0,0 +1,26 @@
+import java.util.ArrayList;
+import java.util.List;
+import java.util.ListIterator;
+
+/* Demonstrates ListIterator, which supports bidirectional movement and
+   methods like hasPrevious() / previous() in addition to next().
+*/
+
+public class ListIteratorDemo {
+    public static void main(String[] args) {
+        List<Integer> list = new ArrayList<>(List.of(5, 10, 50, 60, 70, 80, 90));
+
+        ListIterator<Integer> it = list.listIterator();
+
+        System.out.println("Forward traversal using ListIterator:");
+        while (it.hasNext()) {
+            Integer val = it.next();
+            System.out.println("next(): " + val + "  | hasPrevious(): " + it.hasPrevious());
+        }
+
+        System.out.println("\nBackward traversal using ListIterator:");
+        while (it.hasPrevious()) {
+            System.out.println("previous(): " + it.previous());
+        }
+    }
+}