给你链表的头结点 head ，请将其按 升序 排列并返回 排序后的链表 。 Python怎么输出

可以使用归并排序的思想对链表进行排序，具体实现如下：

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

def sortList(head: ListNode) -> ListNode:
    if not head or not head.next:
        return head
    
    # 快慢指针寻找中点
    slow, fast = head, head.next
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    
    # 分割为两个子链表
    mid = slow.next
    slow.next = None
    left = sortList(head)
    right = sortList(mid)
    
    # 归并两个子链表
    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

输出可以使用以下代码进行测试：

# 构造一个链表 4->2->1->3
head = ListNode(4)
head.next = ListNode(2)
head.next.next = ListNode(1)
head.next.next.next = ListNode(3)

# 排序并输出结果
res = sortList(head)
while res:
    print(res.val, end='->')
    res = res.next

输出结果为：

1->2->3->4->