对单个链表进行排序

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

    # 递归地将链表分成两半
    slow, fast = head, head.next
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    mid = slow.next
    slow.next = None

    # 递归地对两个子链表进行排序
    left, right = sortList(head), sortList(mid)

    # 合并两个子链表
    dummy = ListNode()
    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 or right

    return dummy.next