题解 | #删除有序链表中重复的元素-II#

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 
# @return ListNode类
#
class Solution:
    def deleteDuplicates(self , head: ListNode) -> ListNode:
        # write code here
        # 同样用集合的方法解决
        if not(head):
            return head
        visit = {head.val}
        this = head.next
        # 记录出现过的数字
        vnum = [] 
        while this:
            if this.val in visit:
                vnum.append(this.val)
            else:
                visit.add(this.val)
            this = this.next
        # 删除重复出现的元素
        # 找到第一个不重复的元素以此为头节点
        this = head
        while this:
            if this.val in vnum:
                this = this.next
            else:
                break
        # 若全部都是重复的，则this == None
        if not(this):
            return this
        
        # 找到第一个不重复元素，以此为头节点，并设置pre节点为this节点的前一个节点，方便后续删除节点
        newhead = this
        pre = newhead
        this = this.next
        # 开始删除元素
        while this:
            if this.val in vnum:
                pre.next = this.next
                this = this.next
            else:
                pre = this
                this = this.next
        # 返回新的头节点
        return newhead