Remove Duplicates from Sorted List in Python

Question

There's a leetcode question 83: Given the head of a sorted linked list, delete all duplicates such that each element appears only once. Return the linked list sorted as well.

For example: Input: head = [1,1,2] Output: [1,2]

I am new to linked list and I know there're better solutions. Code1 doesn't work. When input is [1,2,2], the output is [1]. Code2 works, the only difference is the position of "t.next = None". Why doesn't Code1 work? Can someone please explain? Thanks!

Code1

   class Solution(object):
        def deleteDuplicates(self, head):
            """
            :type head: ListNode
            :rtype: ListNode
            """        
            dummy = ListNode()
            t = dummy
            t.val = None
            while head:      
                if t.val != head.val:
                    t.next = head
                    t = t.next
                t.next = None           
                head = head.next   
            return(dummy.next)

Code2

class Solution(object):
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """        
        dummy = ListNode()
        t = dummy
        t.val = None
        while head:
            t.next = None
            if t.val != head.val:
                t.next = head
                t = t.next                
            head = head.next   
        return(dummy.next)

Answer 1

The lines:

if t.val != head.val:
    t.next = head
    t = t.next
t.next = None
head = head.next

are the problem. If t.val != head.val in some iteration, these lines are equivalent to:

t.next = head
t = head
head.next = None
head = head.next  # Same as head = None

which immediately breaks the loop the first time that t.val != head.val.