删除链表中的最后一个节点

Question

I am implementing deletion of last node in linked list using Python. Below is my code:

class Node:
    def __init__(self, key):
        self.key = key
        self.next = None
        
def printList(head):
    curr = head
    while curr != None:
        print(curr.key, end=" ")
        curr = curr.next
    
def deleteLastNode(head):
    if head == None:
        return None
    
    temp = head
    
    # Iterating till the last Node
    while temp.next != None:
        temp = temp.next
    temp = None
    return head

# Driver code
head = Node(10)
head.next = Node(20)
head.next.next = Node(30)

head = deleteLastNode(head)
printList(head)

However, in output I'm still getting the complete linked list.

10 20 30

My doubt is why the last node is not deleted. Also, how is it printing value 30 when the last node is already set to temp = None ?

Answer 1

Well, your linked list is:

10 -> 20 -> 30

^head

Your iteration:

10 -> 20 -> 30

^head       ^temp

Then when you do temp = None , just means(you just assign None to temp ):

10 -> 20 -> 30       None

^head                ^temp

A correct way is when you iterate on 20 , do temp.next = None to remove the reference to the last node. So your code might be:

class Node:
    def __init__(self, key):
        self.key = key
        self.next = None
        
def printList(head):
    curr = head
    while curr != None:
        print(curr.key, end=" ")
        curr = curr.next
    
def deleteLastNode(head):
    if head == None:
        return None
    
    temp = head
    
    # Iterating till the last Node
    while temp.next.next != None:
        temp = temp.next
    temp.next = None
    return head

# Driver code
head = Node(10)
head.next = Node(20)
head.next.next = Node(30)

head = deleteLastNode(head)
printList(head)

This code would work when your linked list contain at least two elements. When there is only one element, this will raise exception. I would recommend you use a dummy head node which next point to the real head node.

Answer 2

#Another way of solving 
class Node:
    def __init__(self, key):
        self.key = key
        self.next = None
    
def printList(head):
    curr = head
    while curr != None:
        print(curr.key, end=" ")
        curr = curr.next

def deleteLastNode(head):
    if head == None:
        return None

    temp = head
    prev=None    #creating the value of previous element
    # Iterating till the last Node
    while temp.next != None:
        prev=temp  #updating for every iteration
        temp = temp.next
    prev.next = None #returning as NONE value
    return head

# Driver code
head = Node(10)
head.next = Node(20)
head.next.next = Node(30)

head = deleteLastNode(head)
printList(head)