input: from flask import Flask, request
  import json
  
  app = Flask(__name__)
  items = ["Apple", "Banana", "Orange"]
  
  @app.route('/items', methods=["GET"])
  def get_items():
      """
      Returns the list of items in JSON format.
      >>> import requests
      >>> response = requests.get('http://localhost:5000/items')
      >>> response.json() == ["Apple", "Banana", "Orange"]
      True
      """
      return json.dumps(items)
  
  @app.route('/items/<item>', methods=["DELETE"])
  def delete_item(item):
      """
      Deletes an item from the list if it exists and returns a success message.
      If the item does not exist, it returns an error message.
      >>> import requests
      >>> response = requests.delete('http://localhost:5000/items/Banana')
      >>> response.json() == {"message": "Item successfully deleted."}
      True
      >>> response = requests.delete('http://localhost:5000/items/Grapes')
      >>> response.json() == {"message": "Item not found."}
      True
      """
      if item in items:
          items.remove(item)
          message = "Item successfully deleted."
      else:
          message = "Item not found."
      return json.dumps({"message": message})
output:     if item in items:
          items.remove(item)
          message = "Item successfully deleted."
      else:
          message = "Item not found."
      return json.dumps({"message": message})