"""
lstack.py　栈的链式存储
重点
思路分析：
1. 基本的实现模型源于　链表
2. 链表的开头作为栈顶 ？ （不用每次遍历）
"""
#　自定义异常类
class StackError(Exception):
  pass

#节点类
class Node:
  def __init__(self, data, next=None):
    self.data = data
    self.next = next

# 栈操作类
class LStack:
  def __init__(self):
    #　定义栈顶位置属性
    self._top = None

  def is_empty(self):
    return self._top is None

  def push(self,elem):
    self._top = Node(elem,self._top)

  #　弹栈
  def pop(self):
    if self._top is None:
      raise StackError("stack is empty")
    val = self._top.data
    self._top = self._top.next
    return val

  #　查看栈顶值
  def top(self):
    if self._top is None:
      raise StackError("stack is empty")
    return self._top.data

if __name__ == "__main__":
  ls = LStack()
  ls.push(10)
  ls.push(20)
  ls.push(30)
  print(ls.top())
  ls.pop()
  print(ls.top())