83. Remove Duplicates from Sorted List

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

class Solution:
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        dummy = ListNode(None)
        dummy.next = head
        previous = dummy
        current = head
        while current:
            if current.val == previous.val:
                previous.next = current.next
            else:
                previous = current
            current = current.next
        return dummy.next