Remove Duplicates from Sorted Linked List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example, Given 1->1->2, return 1->2. Given 1->1->2->3->3, return 1->2->3.

URL: https://leetcode.com/problems/remove-duplicates-from-sorted-list/

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head == None:
            return head
        elif head != None and head.next == None:
            return head
        else:
            lookup = {}
            current = head
            prev = head
            while current != None:
                if current.val in lookup:
                    prev.next = prev.next.next
                else:
                    lookup[current.val] = True
                    prev = current
                current = current.next

            return head