input: 
  from flask import Flask, request
  import json
  
  app = Flask(__name__)
  items = ["Apple", "Banana", "Orange"]
  
  @app.route('/items', methods=["GET"])
  def get_items():
      return json.dumps(items)
  
  @app.route('/items/<item>', methods=["DELETE"])
  def delete_item(item):
      if item in items:
          items.remove(item)
          message = "Item successfully deleted."
      else:
          message = "Item not found."
      return json.dumps({"message": message})
  
  # Test cases:
  # To test the /items/<item> route with an existing item:
  # curl -X DELETE http://127.0.0.1:5000/items/Banana
  # Expected output: {"message": "Item successfully deleted."}
  
  # To test the /items/<item> route with a non-existing item:
  # curl -X DELETE http://127.0.0.1:5000/items/Grapes
  # Expected output: {"message": "Item not found."}
  
output:     
  @app.route('/items', methods=["POST"])
  def add_item():
      new_item = request.json.get('item')
      if new_item and new_item not in items:
          items.append(new_item)
          message = "Item successfully added."
      else:
          message = "Item already exists or is invalid."
      return json.dumps({"message": message})
  
  # Test cases:
  # To test the /items route with a new item:
  # curl -X POST -H "Content-Type: application/json" -d '{"item": "Grapes"}' http://127.0.0.1:5000/items
  # Expected output: {"message": "Item successfully added."}
  
  # To test the /items route with an existing item:
  # curl -X POST -H "Content-Type: application/json" -d '{"item": "Apple"}' http://127.0.0.1:5000/items
  # Expected output: {"message": "Item already exists or is invalid."}
  