input: Write a python function to simulate a stack's push and pop operations using a class.
  assert simulate_stack_operations('Hello World!') == '!dlroW olleH'
  assert simulate_stack_operations('Stack') == 'kcatS'
  assert simulate_stack_operations('Python') == 'nohtyP'
output: def simulate_stack_operations(input_string):
      class Stack:
          def __init__(self):
              self.items = []
  
          def push(self, item):
              self.items.append(item)
  
          def pop(self):
              return self.items.pop() if self.items else None
  
      stack = Stack()
      for char in input_string:
          stack.push(char)
  
      reversed_string = ''
      while stack.items:
          reversed_string += stack.pop()
  
      return reversed_string