class Stack(object):
  """栈"""
  def __init__(self):
     self.items = []
  def is_empty(self):
    """判断是否为空"""
    return self.items == []
  def push(self, item):
    """加入元素"""
    self.items.append(item)
  def pop(self):
    """弹出元素"""
    return self.items.pop()
  def peek(self):
    """返回栈顶元素"""
    return self.items[len(self.items)-1]
  def size(self):
    """返回栈的大小"""
    return len(self.items)
if __name__ == "__main__":
  stack = Stack()
  stack.push("hello")
  stack.push("world")
  stack.push("itcast")
  print stack.size()
  print stack.peek()
  print stack.pop()
  print stack.pop()
  print stack.pop()