leetcode148. Sort List

和数组里面的归并排序相同，用两个指针分别对应low high，递归进行归并排序然后merge把两个链表合在一起

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
    ListNode* mergeSort(ListNode* head)
    {
        if (head->next == NULL)
            return head;
        ListNode* slow;
        ListNode* fast;
        slow = head;
        fast = head;
        while (fast->next&&fast->next->next)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* p;
        p = slow->next;
        slow->next = NULL;
        ListNode* m;
        ListNode* n;
        m = mergeSort(head);
        n = mergeSort(p);
        return merge(m, n);
    }
    ListNode* merge(ListNode* p, ListNode* q)
    {
        ListNode* dummyNode;
        dummyNode = new ListNode();
        ListNode* pre;
        pre = dummyNode;
        while (p&&q)
        {
            if (p->val <= q->val)
            {
                pre->next = new ListNode(p->val);
                pre = pre->next;
                p = p->next;
            }
            else
            {
                pre->next = new ListNode(q->val);
                pre = pre->next;
                q = q->next;
            }
        }
        while (p)
        {
            pre->next = new ListNode(p->val);
            pre = pre->next;
            p = p->next;
        }
        while (q)
        {
            pre->next = new ListNode(q->val);
            pre = pre->next;
            q = q->next;
        }
        ListNode* res;
        res = dummyNode->next;
        delete dummyNode;
        return res;
    }
public:
    ListNode* sortList(ListNode* head) {
        if (head == NULL)
            return NULL;
        return mergeSort(head);
    }
};