83 remove duplicates from sorted list

def (self, head):
    """
    :type head: ListNode
    :rtype: ListNode
    """
    if head == None:
        return None
    p1 = head
    p2 = head.next
    while p2 != None:
        if p1.val != p2.val:
            p1.next = p2
            p1 = p2
        p2 = p2.next
    if p1 != None:
        p1.next = None
    return head