排序链表

def solve(____):
    if not head or not head.next:
        return head
    slow, fast = head, head.next
    while fast and fast.next:
        slow, fast = slow.next, fast.next.next
    mid, slow.next = slow.next, None
    left, right = solve(____), solve(____)
    dummy = cur = ListNode(____)
    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