删除排序链表中的重复元素II --链表

输入: 1->2->3->3->4->4->5
输出: 1->2->5

输入: 1->1->1->2->3
输出: 2->3

public class ListNode {
    public var val: Int?
    public var next: ListNode?
    public init(_ val: Int){
        self.val = val
        self.next = nil
    }
}

func deleteDuplicates(_ head: ListNode?) -> ListNode? {
    if head?.next == nil {
        return head
    }
    var newHead: ListNode? //建立新链表的头部节点
    var newCurrent: ListNode? //新链接当前的值
    var pre: ListNode? //原链表的虚拟头节点
    var current: ListNode? = head //原链表当前访问的节点,默认第一个为头节点
    var nextCurrent: ListNode? = head?.next
    while current != nil {
        if pre?.val != current?.val, current?.val != nextCurrent?.val {
            //判断是否是新链表的第一个元素
            if newHead == nil {
                newHead = current
                newCurrent = current
            } else {
                newCurrent?.next = current
                newCurrent = current
            }
        }
        pre = current
        current = nextCurrent
        nextCurrent = current?.next
    }
    newCurrent?.next = current
    return newHead
}