数据结构 栈（顺序表和链表实现） 回顾练习

#!/usr/bin/env python3

class StackUnderflow(ValueError):
        pass

class Node(object):
        def __init__(self, elem, next_=None):
                self.elem = elem
                self.next = next_

class LStack(object):
        def __init__(self):
                self._elems = []

        def is_empty(self):
                return self._elems == []

        def push(self, elem):
                self._elems.append(elem)

        def pop(self):
                if self.is_empty():
                        raise StackUnderflow
                return self._elems.pop()

        def top(self):
                if self.is_empty():
                        raise StackUnderflow
                return self._elems[-1]

class SStack(object):
        def __init__(self):
                self._top = None

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

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

        def pop(self):
                if self.is_empty():
                        raise StackUnderflow
                e = self._top.elem
                self._top = self._top.next
                return e

        def top(self):
                if self.is_empty():
                        raise StackUnderflow
                return self._top.elem

if __name__ == '__main__':
        """
        ls = LStack()
        ls.push(3)
        ls.push(2)
        print(ls.top())
        print(ls.pop())
        print(ls.pop())
        """
#-----------------------------------
        ss = SStack()
        ss.push(3)
        ss.push(2)
        print(ss.top())
        print(ss.pop())
        print(ss.pop())