题目：
删除排序链表中的重复元素：给定一个排序链表，删除所有重复的元素，使得每个元素只出现一次。
思路：
双指针法。
程序1：暴力法，超时了
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def deleteDuplicates(self, head: ListNode) -> ListNode:
if not head:
return None
myNode = head
while myNode and myNode.next:
if myNode.val == myNode.next.val:
myNode.next = head.next.next
else:
myNode = myNode.next
return head
程序2：双指针
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def deleteDuplicates(self, head: ListNode) -> ListNode:
if not head:
return None
if not head.next:
return head
index1 = head
index2 = head.next
while index2:
if index1.val == index2.val:
index2 = index2.next
elif index1.val != index2.val:
index1.next = index2
index1 = index1.next
index2 = index2.next
index1.next = None
return head