python单链表

#!/usr/bin/env python3
# -*- coding:utf-8 -*-

class LNode:
    """
    结点类
    """

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

class LinkedListUnderflow(ValueError):
    """
    自定义异常
    """
    pass

class LList:
    """
    链表类
    """

    def __init__(self):
        self._head = None

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

    # 操作pop删除表头结点并返回这个结点里的数据
    def pop(self):
        if self._head is None:
            raise LinkedListUnderflow("in pop")
        e = self._head.elem
        self._head = self._head.next
        return e

    # 在表头插入元素
    def prepend(self, elem):
        self._head = LNode(elem, self._head)

    # 在链表最后插入元素
    def append(self, elem):
        if self._head is None:
            self._head = LNode(elem)
            return None
        # 如果链表为空则直接把表头指向需要插入的元素即可在。实际上是在操作_head域

        p = self._head
        while p.next is not None:
            p = p.next
        p.next = LNode(elem)
        # 如果不为空，则先从头扫描链表，找到最后的结点，然后把最后结点的next指向需要插入的元素即可。实际上是在操作next域

    # 删除最后一个结点
    def pop_last(self):
        if self._head is None:  # 空表
            raise LinkedListUnderflow("in pop_last")
        p = self._head
        if p.next is None:  # 如果表长为1，则清空之，并返回原来的元素
            e = p.elem
            self._head = None
            return e
        while p.next.next is not None:
            p = p.next
        e = p.next.elem
        p.next = None
        return e
        # 如果表长大于1,则从头扫描，找到倒数第二个结点，把倒数第二个结点的next域置空，并返回最后一个结点

    def printall(self):  # 扫描打印链表的每个元素
        p = self._head
        while p is not None:
            print(p.elem, end='')
            if p.next is not None:
                print(',', end='')
            p = p.next

    def elements(self):  # 写一个生成器，使链表支持for操作
        p = self._head
        while p is not None:
            yield p.elem
            p = p.next

class LList1(LList):  # 派生一个变形单链表类
    def __init__(self):
        LList.__init__(self)
        self._rear = None

    def prepend(self, elem):  # 重构prepend方法
        if self._rear is None:
            self._head = LNode(elem, self._head)
            self._rear = self._head
        else:
            self._head = LNode(elem, self._head)

    def append(self, elem):
        if self._head is None:
            self._head = LNode(elem, self._head)
            self._rear = self._head
        else:
            self._rear.next = LNode(elem)
            self._rear = self._rear.next

    def pop_last(self):
        if self._head is None:
            raise LinkedListUnderflow("in pop_last")
        p = self._head
        if p.next is None:
            self._head = None
            return p.elem
        while p.next.next is not None:
            p = p.next
        e = p.next.elem
        p.next = None
        self._rear = p
        return e