Python链接列表追加

Question

I made an append method for an UnorderedList() class that works fine in my IDLE window but when its assigned to the University's test of:

my_list = UnorderedList()
my_list.append(13)
for num in my_list: 
    print(num, end=" ")
print()    

it returns an error: AttributeError: Nonetype object has no attribute 'getNext' .

Here is the append method:

def append(self,item):
    current = self.head
    while current.getNext() != None:
        current = current.getNext()
    current.setNext(Node(item))

Here is the rest of my classes and code:

class Node:
    def __init__(self,initdata):
        self.data = initdata
        self.next = None

    def getData(self):
        return self.data

    def getNext(self):
        return self.next

    def setData(self,newdata):
        self.data = newdata

    def setNext(self,newnext):
        self.next = newnext

class UnorderedList:

    def __init__(self):
        self.head = None
        self.count = 0

    def append(self,item):
        current = self.head
        while current.getNext() != None:
            current = current.getNext()
        current.setNext(Node(item))

Why is the test returning that error and how I can fix my append method?

Answer 1

The problem is here in the append method:

def append(self,item):
    current = self.head
    while current.getNext() != None:
        current = current.getNext()
    current.setNext(Node(item))

In the first iteration, the value of current is self.head , which is set to None initially, and you don't check for it.

So instead, alter this and introduce checks for this condition ad below:

def append(self,item):
    current = self.head
    if current:
        while current.getNext() != None:
            current = current.getNext()
        current.setNext(Node(item))
    else:
        self.head = Node(item)

PS: you are also using a variable self.count , which you are not updating. You might want to update the same as well.

Answer 2

your append method works fine but it traverses the list until it finds the last node - which makes it O(n). If you keep track of the last node, you can make an append which is O(1):

def append_O1(self, item):
        temp = Node(item)
        last = self.tail
        last.setnext(temp)
        self.tail = temp
        self.length += 1

To enable this your list class constructor should be:

class unorderedList:
    def __init__(self):
        self.head = None
        self.tail = None
        self.length = 0

Answer 3

Adding little more elaborated append methods

Method to insert at the beginning

def inserAtBeginning(self, item):
    newNode = Node(item)
    newNode.setdata(item)
    if self.listLength() == 0:
        self.head = newNode
    else:
        newNode.setnext(self.head)
        self.head = newNode

Method to insert at the end

def insertAtEnd(self, item):
    newNode = Node(item)
    newNode.setdata(item)
    current = self.head
    while current.getnext() != None:
        current = current.getnext()
    current.setnext(newNode)

Method to insert at the specified position

def insertAtPos(self, pos, item):
    if pos > self.listLength() or pos < 0:
        return None
    if pos == 0:
        self.inserAtBeginning(item)
    else:
        if pos == self.listLength():
            self.insertAtEnd(item)
        else:
            newNode = Node(item)
            newNode.setdata(item)
            current = self.head
            count = 0
            while count < pos - 1:
                count += 1
                current = current.getnext()
            newNode.setnext(current.getnext())
            current.setnext(newNode)

Answer 4

This O(n) implementation should work for an empty list too :

def append(self,item):
    current = self.head
    if current == None:
        self.head = Node(item)
    else:
        while current.getNext() != None:
            current = current.getNext()

        current.setNext(Node(item))