input: Write a python function to implement a stack with methods to check if it's empty, push an item, pop an item, and peek at the top item.
  assert s.isEmpty() == True
  s.push(1)
  assert s.isEmpty() == False
  s.push(2)
  assert s.peek() == 2
  assert s.pop() == 2
  assert s.pop() == 1
  assert s.isEmpty() == True
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
  
  # Usage
  s = Stack()
  assert s.isEmpty() == True
  s.push(1)
  assert s.isEmpty() == False
  s.push(2)
  assert s.peek() == 2
  assert s.pop() == 2
  assert s.pop() == 1
  assert s.isEmpty() == True