Sort List

ListNode *listSort(ListNode *head)
{
//定义快慢指针，找到链表中心
ListNode *slow=head,*fast=head;
while (fast->next!=nullptr&&fast->next->next!=nullptr)
{
slow = slow->next;
fast = fast->next->next;
}
//分为两段
fast = slow;
slow = slow->next;
fast->next = nullptr;

ListNode *l1=listSort(head);
ListNode *l2=listSort(slow);

return mergeList(l1, l2);
}