input: Write a python function to simulate a stack using a class implementation, and include methods for pushing, popping, peeking, checking size, and checking if the stack is empty.
  assert s := Stack(); s.push(1); s.push(2); s.push(3); s.size() == 3
  assert s.pop() == 3
  assert s.peek() == 2
  assert s.size() == 2
  assert s.isEmpty() == False
  assert s.pop() == 2; s.pop() == 1; s.isEmpty() == True
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 peek(self):
          if not self.isEmpty():
              return self.items[-1]
          return None
  
      def size(self):
          return len(self.items)
  
      def isEmpty(self):
          return len(self.items) == 0