How to delete and return the last item in a linked list?

Question

My home work needs me to pop the last item from the linked list and for some reason, some test cases works but some did not and I don't know why.

class Node:
    def __init__(self, init_data):
        self.data = init_data
        self.next = None
    def get_data(self):
        return self.data
    def get_next(self):
        return self.next
    def set_data(self, new_data):
        self.data = new_data
    def set_next(self, new_next):
        self.next = new_next
    def __str__(self):
        return str(self.data)

class LinkedList:
    def __init__(self):
        self.head = None
    def add(self, item):
        new_node = Node(item)
        new_node.set_next(self.head)
        self.head = new_node

    def __str__(self):
        result = "("
        node = self.head
        if node != None:
            result += str(node.data)
            node = node.next
            while node:
                result += ", " + str(node.data)
                node = node.next
        result += ")"
        return result
    def remove_from_tail(self):
        if self.head is None:
            return None
        prev = None
        cur = self.head
        while cur.next is not None:
            prev = cur
            cur = cur.next
        if prev:
            prev.next = None
        return cur

#test case one is incorrect
my_list = LinkedList()
my_list.add(400)
print(my_list.remove_from_tail())
my_list.add(300)
print(my_list.remove_from_tail())
my_list.add(200)
print(my_list.remove_from_tail())
my_list.add(100)
print(my_list.remove_from_tail())
#should be 400 300 200 100 but instead I got 400 400 300 200

#test case two works fine
fruit = LinkedList()
fruit.add('cherry')
fruit.add('banana')
fruit.add('apple')
last_one = fruit.remove_from_tail()
print('Removed:', last_one)#got"Removed: cherry"
print(fruit)#(apple, banana)

I don't know what is the reason for test case one to fail when cur = self.head and self.head should point to 300 after 400 is removed. So when I return cur it shouldn't print out two 400. Any help would be appreciated.

Answer 1

Your code is fine, the problem is that you are calling remove_from_tail after every add, but what you want to do is call add till you add all elements, and then call remove_from_tail one by one, which works fine

my_list = LinkedList()
my_list.add(400)
my_list.add(300)
my_list.add(200)
my_list.add(100)
print(my_list.remove_from_tail())
print(my_list.remove_from_tail())
print(my_list.remove_from_tail())
print(my_list.remove_from_tail())

This outputs

400
300
200
100

Also your add function is a little odd, generally we fix the head and for every new element, traverse to the end of the list and append it, but in your add you are updating every element on head and updating head.

Whereas your remove_from_tail assumes that the head is fixed and we are traversing till the end and then updating, which actually happens

So after you run add 4 times, the list is

head
100 -> 200 -> 300 -> 400

And as you expect, the tail is removed accordingly,from 400 to 100

Also in your remove_from_tail function, you need to take care of the case where only one element remains in the list, and you want to remove it, once you add that your original test case works too

def remove_from_tail(self):
    #If linked list is empty return None
    if self.head is None:
        return None
    #If only one element in list, return that element and set head to None
    elif self.head.next is None:
        cur = self.head
        self.head = None
        return cur

    #Otherwise iterate through the list
    prev = None
    cur = self.head
    while cur.next is not None:
        prev = cur
        cur = cur.next
    if prev:
        prev.next = None
    return cur

The original test case then prints

400
300
200
100

as well