如何根据值对ListNode (链接)进行排序？

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

# 归并排序
def mergeSort(head):
    if not head or not head.next:
        return head
    
    # 快慢指针找到中间节点
    fast, slow = head, head
    pre = None
    while fast and fast.next:
        pre = slow
        fast = fast.next.next
        slow = slow.next
    pre.next = None
    
    # 递归地对两个子链表进行排序
    left = mergeSort(head)
    right = mergeSort(slow)
    
    # 合并两个有序链表
    return merge(left, right)

# 合并两个有序链表
def merge(left, right):
    dummy = ListNode(0)
    cur = dummy
    while left and right:
        if left.val < right.val:
            cur.next = left
            left = left.next
        else:
            cur.next = right
            right = right.next
        cur = cur.next
    cur.next = left if left else right
    
    return dummy.next

# 创建链表
head = ListNode(4)
node1 = ListNode(2)
node2 = ListNode(1)
node3 = ListNode(3)
head.next = node1
node1.next = node2
node2.next = node3

# 调用排序函数
sorted_head = mergeSort(head)

# 打印排序结果
while sorted_head:
    print(sorted_head.val)
    sorted_head = sorted_head.next