simplify the code

class Node: def __init__(self, data=None): self.data = data self.next = None def is_valid_expression(expression): stack = [] for char in expression: if char == '(': stack.append(char) elif char == ')': if not stack: return False stack.pop() return not stack def infix_to_postfix(expression): result = [] stack = [] for char in expression: if char.isalnum(): result.append(char) elif char == '(': stack.append(char) elif char == ')': while stack and stack[-1] != '(': result.append(stack.pop()) stack.pop() else: while stack and (stack[-1] == '+' or stack[-1] == '-'): result.append(stack.pop()) stack.append(char) while stack: result.append(stack.pop()) return ''.join(result) def expression_to_linked_list(expression): linked_list = Node("0") current = linked_list if is_valid_expression(expression): postfix_expression = infix_to_postfix(expression) for char in postfix_expression: current.next = Node(char) current = current.next return linked_list.next # Skipping the initial "0" node # Example usage: user_expression = input("Enter a mathematical expression: ") resulting_linked_list = expression_to_linked_list(user_expression) if resulting_linked_list: print("Postfix Linked List:") current = resulting_linked_list while current: print(current.data, end=" ") current = current.next else: print("Invalid expression with mismatched parentheses.")