feat(StackDemo1): add interactive stack input and save contents to file

What
- Added StackDemo1 class.
- Used Scanner to take user input.
- Created a Stack<Integer>.
- Asked user how many elements to push.
- Iteratively read integers and pushed them onto the stack.
- Printed the stack contents.
- Implemented saveStackToFile() method:
  - Used BufferedWriter + FileWriter to write each stack element to a file, one per line.
  - Wrapped file writing in try-with-resources for safe closure.
- Closed Scanner after input.

Why
- Demonstrates interactive usage of Stack with user input.
- Shows how to persist stack contents into a file.
- Combines collections (Stack) with I/O (BufferedWriter) for practical example.

How
- User prompted for n.
- For loop reads n integers via sc.nextInt() and pushes into stack.
- After input, printed stack.
- saveStackToFile() iterates stack, writes each value with newline.
- File saved to hardcoded path.

Logic
- Inputs: integer count n and stack elements.
- Outputs:
  - Printed stack contents.
  - Saved stack elements to text file.
- Flow:
  1. Prompt for size.
  2. Read elements, push to stack.
  3. Print stack contents.
  4. Write contents to file.
- Edge cases:
  - Non-integer input throws InputMismatchException.
  - File path invalid or lacking permissions → IOException caught and error message shown.
  - Empty stack still creates empty file.
- Complexity:
  - Stack push O(1) per element.
  - File write O(n).
- Concurrency: Stack is synchronized but Scanner/FileWriter are not thread-safe here.

Real-life applications
- Capturing runtime stack input from users.
- Persisting stack state for later retrieval or debugging.
- Useful template for combining data structures with file I/O.

Notes
- Hardcoded file path may fail across systems; should use relative paths or configurable paths.
- Prefer try-with-resources for Scanner as well to ensure closure.
- In modern code, ArrayDeque often replaces Stack, but demo illustrates legacy class + file persistence nicely.

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

Author: Someshdiwan

Section 25 Collections Frameworks/List Interface/Vector/Stack/src/Output.txt

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