链表的Mergesort的Python实现不起作用

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

def mergeSort(head):
    if not head or not head.next:
        return head
    
    # 找到链表的中点
    slow, fast = head, head.next
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    
    # 将链表分为两部分
    mid = slow.next
    slow.next = None
    
    # 递归地对两部分链表进行排序
    left = mergeSort(head)
    right = mergeSort(mid)
    
    # 合并两个有序链表
    dummy = ListNode(0)
    curr = dummy
    while left and right:
        if left.val < right.val:
            curr.next = left
            left = left.next
        else:
            curr.next = right
            right = right.next
        curr = curr.next
    
    curr.next = left if left else right
    
    return dummy.next