answer:Here are the three versions of the Java code to reverse a string: **Version 1: Recursive Version** ```java public class Main { public static void main(String[] args) { String str = "Hello, World!"; System.out.println("Original String: " + str); System.out.println("Reversed String: " + reverseString(str)); } public static String reverseString(String str) { if (str.isEmpty()) return str; else return reverseString(str.substring(1)) + str.charAt(0); } } ``` In this version, we define a recursive function `reverseString` that takes a string `str` as input. The base case for the recursion is when the string is empty. If the string is not empty, we recursively call `reverseString` on the substring of `str` starting from the second character to the end, and append the first character of `str` to the result. **Version 2: Iterative Version** ```java public class Main { public static void main(String[] args) { String str = "Hello, World!"; System.out.println("Original String: " + str); System.out.println("Reversed String: " + reverseString(str)); } public static String reverseString(String str) { StringBuilder reversed = new StringBuilder(); for (int i = str.length() - 1; i >= 0; i--) { reversed.append(str.charAt(i)); } return reversed.toString(); } } ``` In this version, we use a `StringBuilder` to efficiently build the reversed string. We iterate over the input string from the end to the beginning and append each character to the `StringBuilder`. Finally, we return the reversed string as a string. **Version 3: Version Using `StringBuilder` Class** ```java public class Main { public static void main(String[] args) { String str = "Hello, World!"; System.out.println("Original String: " + str); System.out.println("Reversed String: " + reverseString(str)); } public static String reverseString(String str) { StringBuilder sb = new StringBuilder(str); return sb.reverse().toString(); } } ``` In this version, we create a `StringBuilder` object from the input string and use the `reverse` method to reverse the string. This is the most concise and efficient way to reverse a string in Java. Note that the recursive version may cause a `StackOverflowError` for very long strings, while the iterative versions do not have this issue. Therefore, the iterative versions are generally preferred.

answer:```python import requests from bs4 import BeautifulSoup import urllib.parse def fetch_and_parse(url): try: response = requests.get(url) response.raise_for_status() # Raise an exception for HTTP errors soup = BeautifulSoup(response.text, 'html.parser') return soup except requests.exceptions.RequestException as e: print(f"Request failed: {e}") return None def main(): url = input("Enter the URL: ") try: result = urllib.parse.urlparse(url) if not all([result.scheme, result.netloc]): print("Invalid URL") else: soup = fetch_and_parse(url) if soup: print(soup.prettify()) except ValueError as e: print(f"Invalid URL: {e}") if __name__ == "__main__": main() ``` This script takes a URL as input from the user, attempts to fetch the HTML content, and then parses it using BeautifulSoup. If the request is successful, it prints the parsed HTML content. It handles potential exceptions such as invalid URLs, request failures, and HTTP errors. The `urllib.parse.urlparse` function is used to validate the URL input by the user.

answer:Here's your code for a queue data structure implemented in Python using a linked list with the specified methods. ```python class Node: def __init__(self, data): self.data = data self.next = None class Queue: def __init__(self): self.front = None self.rear = None self.num_nodes = 0 def enqueue(self, data): new_node = Node(data) if self.rear: self.rear.next = new_node self.rear = new_node else: self.front = new_node self.rear = new_node self.num_nodes += 1 def dequeue(self): if self.is_empty(): raise IndexError("Cannot dequeue from an empty queue") temp = self.front self.front = self.front.next if self.front is None: self.rear = None self.num_nodes -= 1 return temp.data def is_empty(self): return self.front is None def size(self): return self.num_nodes def peek(self): if self.is_empty(): raise IndexError("Cannot peek an empty queue") return self.front.data # Example usage: queue = Queue() queue.enqueue(1) queue.enqueue(2) queue.enqueue(3) print(queue.dequeue()) # Output: 1 print(queue.size()) # Output: 2 print(queue.peek()) # Output: 2 ``` This implementation creates a `Queue` class using a linked list to store elements. The `enqueue` method adds elements to the end of the queue, and the `dequeue` method removes elements from the front. The `is_empty` method checks if the queue is empty, the `size` method returns the number of elements, and the `peek` method returns the front element without removing it.

answer:Here's your code for a stack data structure implemented in Python using a linked list with the specified methods. ```python class Node: def __init__(self, data): self.data = data self.next = None class Stack: def __init__(self): self.top = None self.num_nodes = 0 def push(self, data): new_node = Node(data) new_node.next = self.top self.top = new_node self.num_nodes += 1 def pop(self): if self.is_empty(): raise IndexError("Cannot pop from an empty stack") temp = self.top self.top = self.top.next self.num_nodes -= 1 return temp.data def is_empty(self): return self.top is None def size(self): return self.num_nodes def peek(self): if self.is_empty(): raise IndexError("Cannot peek an empty stack") return self.top.data # Example usage: stack = Stack() stack.push(1) stack.push(2) stack.push(3) print(stack.pop()) # Output: 3 print(stack.size()) # Output: 2 print(stack.peek()) # Output: 2 ``` This implementation creates a `Stack` class using a linked list to store elements. The `push` method adds elements to the top of the stack, and the `pop` method removes elements from the top. The `is_empty` method checks if the stack is empty, the `size` method returns the number of elements, and the `peek` method returns the top element without removing it.