链式存储(数据结构--栈)

备注:约定以栈的头为栈顶
 1 """
 2 栈的链式存储
 3 重点代码
 4 """
 5 
 6 #　自定义栈异常
 7 class StackError(Exception):
 8     pass
 9 
10 # 创建结点类
11 class Node(object):
12     def __init__(self, val, next=None):
13         self.val = val  # 有用数据
14         self.next = next
15 
16 class LStack:
17     def __init__(self):
18         #　标记栈顶位置
19         self._top = None
20 
21     def is_empty(self):
22         return self._top is None
23 
24     #入栈
25     def push(self,elem):
26         self._top = Node(elem,self._top)
27     
28     #弹栈
29     def pop(self):
30         if self._top is None:
31             raise StackError("stack is empty")
32         p = self._top
33         self._top = p.next
34         return p.val
35 
36     #　查看栈顶元素值
37     def top(self):
38         if self._top is None:
39             raise StackError("stack is empty")
40         return self._top.val
41 
42 if __name__ == "__main__":
43     st = LStack()
44     print(st.is_empty()) #　判断是否为空
45     st.push(10)
46     st.push(20)
47     print(st.top()) #　获取栈顶元素值
48     st.push(30)
49     while not st.is_empty():
50         print(st.pop())