input: Write a python function to implement a stack and perform basic operations like push, pop, isEmpty, peek, and size.
  assert stack.isEmpty() == True
  stack.push(1)
  stack.push(2)
  assert stack.peek() == 2
  assert stack.size() == 2
  assert stack.pop() == 2
  assert stack.size() == 1
  assert stack.isEmpty() == False
output: class Stack:
      def __init__(self):
          self.items = []
  
      def isEmpty(self):
          return len(self.items) == 0
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          return self.items.pop() if not self.isEmpty() else None
  
      def peek(self):
          return self.items[-1] if not self.isEmpty() else None
  
      def size(self):
          return len(self.items)
  
  # Example usage:
  stack = Stack()
  assert stack.isEmpty() == True
  stack.push(1)
  stack.push(2)
  assert stack.peek() == 2
  assert stack.size() == 2
  assert stack.pop() == 2
  assert stack.size() == 1
  assert stack.isEmpty() == False