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

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

class Solution:
    # @param head, a ListNode
    # @return a ListNode
    def deleteDuplicates(self, head):
        if None == head or None == head.next:
            return head

        cur = head
        while None != cur:
            if None != cur.next and cur.val == cur.next.val:
                cur.next = cur.next.next
                continue
            else:
                cur = cur.next

        return head