input: Write a python function to implement a Stack class with methods to push, pop, check if it's empty, peek at the top item, and check the size of the stack.
  assert stack.isEmpty() == True
  stack.push(1)
  stack.push(2)
  assert stack.isEmpty() == False
  assert stack.pop() == 2
  assert stack.peek() == 1
  assert stack.size() == 1
output: class Stack:
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          if not self.isEmpty():
              return self.items.pop()
          return None
  
      def isEmpty(self):
          return len(self.items) == 0
  
      def peek(self):
          if not self.isEmpty():
              return self.items[-1]
          return None
  
      def size(self):
          return len(self.items)