【Sort List】cpp

/**
 * 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 || !head->next ) return head;
        ListNode dummy(-1);
        dummy.next = head;
        ListNode *p1=&dummy, *p2=&dummy;
        while ( p2 && p2->next && p2->next->next )
        {
            p1 = p1->next;
            p2 = p2->next->next;
        }
        ListNode *h1 =  Solution::sortList(p1->next);
        p1->next = NULL;
        ListNode *h2 =  Solution::sortList(dummy.next);
        return Solution::mergeTwo(h1, h2);
    }
    static ListNode* mergeTwo(ListNode *h1, ListNode *h2)
    {
        ListNode dummy(-1);
        ListNode *p = &dummy;
        while ( h1 && h2 )
        {
            if ( h1->val<h2->val )
            {
                p->next = h1;
                h1 = h1->next;
            }
            else
            {
                p->next = h2;
                h2 = h2->next;
            }
            p = p->next;
        }
        p->next = h1 ? h1 : h2;
        return dummy.next;
    }
};