1 a = 10
2 b = 20
3 a,b = b,a

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @Date  : 2018-06-12 11:23:21
# @Author : yudanqu (943775910@qq.com)
# @Link  : https://www.cnblogs.com/yudanqu/
# @Version : $Id$


class Node(object):
  """节点"""

  def __init__(self, elem):
    self.elem = elem
    self.next = None # 初始设置下一节点为空

'''
上面定义了一个节点的类，当然也可以直接使用python的一些结构。比如通过元组（elem, None）
'''


# 下面创建单链表，并实现其应有的功能


class SingleLinkList(object):
  """单链表"""

  def __init__(self, node=None): # 使用一个默认参数，在传入头结点时则接收，在没有传入时，就默认头结点为空
    self.__head = node

  def is_empty(self):
    '''链表是否为空'''
    return self.__head == None

  def length(self):
    '''链表长度'''
    # cur游标，用来移动遍历节点
    cur = self.__head
    # count记录数量
    count = 0
    while cur != None:
      count += 1
      cur = cur.next
    return count

  def travel(self):
    '''遍历整个列表'''
    cur = self.__head
    while cur != None:
      print(cur.elem, end=' ')
      cur = cur.next
    print("\n")

  def add(self, item):
    '''链表头部添加元素'''
    node = Node(item)
    node.next = self.__head
    self.__head = node

  def append(self, item):
    '''链表尾部添加元素'''
    node = Node(item)
    # 由于特殊情况当链表为空时没有next，所以在前面要做个判断
    if self.is_empty():
      self.__head = node
    else:
      cur = self.__head
      while cur.next != None:
        cur = cur.next
      cur.next = node

  def insert(self, pos, item):
    '''指定位置添加元素'''
    if pos <= 0:
        # 如果pos位置在0或者以前，那么都当做头插法来做
      self.add(item)
    elif pos > self.length() - 1:
      # 如果pos位置比原链表长，那么都当做尾插法来做
      self.append(item)
    else:
      per = self.__head
      count = 0
      while count < pos - 1:
        count += 1
        per = per.next
      # 当循环退出后，pre指向pos-1位置
      node = Node(item)
      node.next = per.next
      per.next = node

  def remove(self, item):
    '''删除节点'''
    cur = self.__head
    pre = None
    while cur != None:
      if cur.elem == item:
        # 先判断该节点是否是头结点
        if cur == self.__head:
          self.__head = cur.next
        else:
          pre.next = cur.next
        break
      else:
        pre = cur
        cur = cur.next

  def search(self, item):
    '''查找节点是否存在'''
    cur = self.__head
    while not cur:
      if cur.elem == item:
        return True
      else:
        cur = cur.next
    return False


if __name__ == "__main__":

    # node = Node(100) # 先创建一个节点传进去

  ll = SingleLinkList()
  print(ll.is_empty())
  print(ll.length())

  ll.append(3)
  ll.add(999)
  ll.insert(-3, 110)
  ll.insert(99, 111)
  print(ll.is_empty())
  print(ll.length())
  ll.travel()
  ll.remove(111)
  ll.travel()