#单链表插入和删除一个节点的伪代码算法
#单链表插入一个节点
Write 'a1 a2 a3 ... ai-1 ai+1 ... an-1 an'
Read SingleLinkList
Set ai-1 as node P
Set ai+1 as node Q(#P——>next == Q)
Set ai as node t
Set P——>next to t
Set t——>next to Q
#单链表删除一个节点
Write 'a1 a2 a3 ... ai-1 ai+1 ... an-1 an'
Read SingleLinkList
Set ai-1 as node P
Set ai as node t
Set ai+1 as node Q(#P——>next == t,  t——>next == Q)
Set P——>next = Q

#python中实现单链表插入和删除一个节点
#创建一个节点
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'''
# 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，per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
        per = 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:
                    per.next = cur.next
                break
            else:
                per = 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
l = SingleLinkList()
print(l.length())
l.append(1)
l.add(2)
l.insert(0, 3)
l.insert(1, 4)
l.travel()
l.remove(4)
l.travel()