Sort List(归并排序)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* sortList(ListNode* head) {

        if(head == NULL || head->next ==NULL)            //递归停止条件
        {
            return head;
        }
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast->next && fast->next->next)
        {
            fast = fast->next->next;
            slow = slow->next;
        }

        ListNode* mid = slow->next;
        slow->next = NULL;                  //very important
        ListNode* left = sortList(head);
        ListNode* right = sortList(mid);

        return mergeList(left, right);

    }

    ListNode* mergeList(ListNode* head, ListNode* mid)
    {
        ListNode* p = new ListNode(0);      //定义一个虚拟头节点
        ListNode* p1 = p;
        while(head && mid)
        {
            if(head->val < mid->val)
            {
                p1->next = head;
                head = head->next;
            }
            else
            {
                p1->next = mid;
                mid = mid->next;
            }
            p1 = p1->next;
        }
        if(head)
        {
            p1->next = head;
        }
        else if(mid)
        {
            p1->next = mid;
        }

        return p->next;
    }
};