在 Append 之后获取 Position 并在 Python 的链表中插入

Question

I'm a beginner self-taught programmer and I am following a course on Data Structures. I've run into a snag when trying to call a get_position method after I've run append and insert methods and I don't understand why it's happening. Referencing the code below, print(ll.get_position(5).value) prints "5" before the insert method is called and after insertion, I would expect print(ll.get_position(6).value) would print "5" but it prints "3". Additionally I can pass any number as an argument in the get_position method and it will still print "3" or "4" even if the number is outside the bounds of the linked list. I'm assuming the while loop of the insert method is stuck? How can I reattach e5 to the linked list? Why is it getting lost?

Thank you!

class Element(object):
    def __init__(self, value):
        self.value = value
        self.next = None


class LinkedList(object):
    def __init__(self, head=None):
        self.head = head

    def append(self, new_element):
        current = self.head
        if self.head:
            while current.next:
                current = current.next
            current.next = new_element
        else:
            self.head = new_element

    def get_position(self, position):

        counter = 1
        current = self.head

        while current and counter <= position:
            if counter == position:
                return current

            current = current.next
            counter += 1

    def insert(self, new_element, position):

        counter = 1
        current = self.head

        if position < 1:
            return None

        elif position == 1:
            new_element.next = self.head
            self.head = new_element

        while current and counter < position:
            if counter == position - 1:
                new_element.next = current.next
                current.next = new_element
                return

            current = current.next
            counter += 1


#  Elements in list
e1 = Element(1)
e2 = Element(2)
e3 = Element(3)
e4 = Element(4)
e5 = Element(5)

# Linked List Setup
ll = LinkedList(e1)
ll.append(e2)
ll.append(e3)
ll.append(e4)
ll.append(e5)

# Should print 5
print(ll.get_position(5).value)

ll.insert(e4, 3)

# Expected Ouput: 1,2,4,3,4
print(ll.get_position(1).value)
print(ll.get_position(2).value)
print(ll.get_position(3).value)
print(ll.get_position(4).value)
print(ll.get_position(5).value)
# Expected output: 5 (Actual Output 3)
print(ll.get_position(6).value)
# Expected output: Error. (Actual Output 4)
print(ll.get_position(7).value)

Answer 1

You are using the existing e4 node which is pointing to node e5. create a new element with value 4 and then pass it to insert method.

new_e4 = Element(4)
l1.insert(new_e4, 3)

After insertion of e4 you are expecting the linked list to look like

1 -> 2 -> 4 -> 3 -> 4 -> 5

But actually the linked list has become:

1 -> 2 -> 4 <=> 3 5 ( 5 is lost and 3 is pointing back to 4 and hence infinite loop)