链表排序(归并排序法)

ListNode *sortList(ListNode *head) {
    //链表归并排序
    if(head == nullptr || head->next == nullptr)
        return head;
    else{
        //快慢指针找到中间节点
        ListNode *fast = head,*slow = head;
        while(fast->next != nullptr && fast->next->next != nullptr){
            fast = fast->next->next;
            slow = slow->next;
        }
        fast = slow;
        slow = slow->next;
        fast->next = NULL;
        fast = sortList(head);//前半段排序
        slow = sortList(slow);//后半段排序
        return merge(fast,slow);
    }

}
// merge two sorted list to one
ListNode *merge(ListNode *head1, ListNode *head2)
{
    if(head1 == nullptr)
        return head2;
    if(head2 == nullptr)
        return head1;
    ListNode *res , *p ;
    if(head1->val < head2->val){
        res = head1; 
        head1 = head1->next;
    }else{
        res = head2; 
        head2 = head2->next;
    }
    p = res;
    while(head1 != nullptr && head2 != nullptr){
        if(head1->val < head2->val){
            p->next = head1;
            head1 = head1->next;
        } else{
            p->next = head2;
            head2 = head2->next;
        }
        p = p->next;
    }
    if(head1 != nullptr)
        p->next = head1;
    else if(head2 != nullptr)
        p->next = head2;
    return res;
}