思想：

由於連結串列是排序的，只要判斷head和head.next的val值是否相等，若相等則head.next = head.next.next 反之head = head.next

為了返回，將head暫存dummy

# 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= head 
        
        while head and head.next:
            if head.val == head.next.val:
                head.next = head.next.next
            else:
                head = head.next
        return dummy