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.
題意：排序的連結串列刪除重複的元素，保證每個元素出現一次
runtime:68ms
總結：val不同的，移動指標p，val相同的,指標p不變，移動p.next;
每一次比較p.next.val和p.val之前，都要先判斷p.next存在與否，否則p.next=None時，p.next.val會報錯

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