input: Write a function to implement a stack data structure with push, pop, and size methods.
  assert stack.size() == 0
  stack.push(1)
  stack.push(5)
  assert stack.size() == 2
  assert stack.pop() == 5
  assert stack.size() == 1
  assert stack.pop() == 1
  assert stack.size() == 0
output: class Stack:
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if not self.items:
              return None
          return self.items.pop()
  
      def size(self):
          return len(self.items)
  
  stack = Stack()