Remove Duplicates from Sorted 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.

# 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 not head:
            return head
        pre = ListNode(0)
        pre.next = head
        while head.next:
            if head.val == head.next.val:
                head.next = head.next.next
            else:
                head = head.next
        return pre.next