归并

class Solution
{
public:
    ListNode *sortList(ListNode *head)
    {
        if (head == NULL || head->next == NULL)
        {
            return head;
        }
        return mergeSort(head);
    }
    ListNode *mergeSort(ListNode *head)
    {
        if (head == NULL)
        {
            return NULL;
        }
        if (head->next == NULL)
        {
            return head;
        }
        ListNode *slow = head;
        ListNode *fast = head->next;
        while (fast && fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode *r_head = slow->next;
        slow->next = NULL;
        ListNode *left = mergeSort(head);
        ListNode *right = mergeSort(r_head);
        return mergeTogether(left, right);
    }
    ListNode *mergeTogether(ListNode *head1, ListNode *head2)
    {
        if (head1 == NULL)
        {
            return head2;
        }
        if (head2 == NULL)
        {
            return head1;
        }
        ListNode *root = new ListNode(0);
        ListNode *h = root;
        while (head1 && head2)
        {
            if (head1->val < head2->val)
            {
                h->next = head1;
                head1 = head1->next;
            }
            else
            {
                h->next = head2;
                head2 = head2->next;
            }
            h = h->next;
        }
        if (head1)
        {
            h->next = head1;
        }
        if (head2)
        {
            h->next = head2;
        }
        return root->next;
    }
};

快排

void quickSort(ListNode *head, ListNode *tail)
{
    if(head==tail||head->next==tail)
    {
        return;
    }
    int pivot = head->val;
    ListNode *left = head;
    ListNode *cur = head->next;
    while(cur!=tail)
    {
        if(cur->val<pivot)
        {
            left = left->next;
            swap(left->val, cur->val);
        }
        cur = cur->next;
    }
    swap(left->val, head->val);
    quickSort(head, left);
    quickSort(left->next, tail);
}
ListNode *sortList(ListNode *head)
{
    if (head == NULL || head->next == NULL)
    {
        return head;
    }
    quickSort(head, NULL);
    return head;
}