如何从未排序的链表中删除重复项

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def deleteDuplicates(head):
    if not head:
        return head
    
    # 创建一个哈希表
    visited = set()
    
    # 创建一个虚拟头节点
    dummy = ListNode(0)
    dummy.next = head
    
    prev = dummy
    curr = head
    
    while curr:
        if curr.val in visited:
            # 删除重复节点
            prev.next = curr.next
        else:
            # 将节点值添加到哈希表中
            visited.add(curr.val)
            prev = curr
        
        curr = curr.next
    
    return dummy.next