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 。
找到中点，虚拟头节点存储新链表，重排链表.
func sortList(head *ListNode) *ListNode {
if head == nil || head.Next == nil {
return head
}
slow, fast := head, head.Next
for fast != nil && fast.Next != nil {
slow = slow.Next
fast = fast.Next.Next
}
mid := slow.Next
slow.Next = nil
left := sortList(head)
right := sortList(mid)
return merge(left, right)
}
func merge(l1, l2 *ListNode) *ListNode {
dummy := &ListNode{}
cur := dummy
for l1 != nil && l2 != nil {
if l1.Val < l2.Val {
cur.Next = l1
l1 = l1.Next
} else {
cur.Next = l2
l2 = l2.Next
}
cur = cur.Next
}
if l1 != nil {
cur.Next = l1
} else {
cur.Next = l2
}
return dummy.Next
}