Remove Duplicates from Sorted List

题目：

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.

代码：

此题简单，但未一次通过。

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


class Solution:
    # @param {ListNode} head
    # @return {ListNode}
    def deleteDuplicates(self, head):
        p = head
        if head==None:return head
        while p.next:
            if p.val==p.next.val:
                p.next=p.next.next
            else:
                p = p.next
        return head