leetcode 148. Sort List

https://leetcode.com/problems/sort-list/discuss/46714/Java-merge-sort-solution

链表初始化代码：ListNode* origin = new ListNode (0);

此题要求时间复杂度是o(nlogn)，空间复杂度是o(1)，快排最高时间复杂度可能到达o(n2)，所以用归并排序，数组的归并排序，还需要o(n)的空间复杂度，但链表不用

归并排序是先划分再融合

https://www.cnblogs.com/chengxiao/p/6194356.html图解归并排序

注意的点：因为要拆分成两个链表，所以一定要从中间那个值一定要加null

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast->next && fast->next->next){
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* newhead = slow->next;
        slow->next = NULL;
        ListNode* p1 = sortList(head);
        ListNode* p2 = sortList(newhead);
        return merge(p1,p2);
    }
    ListNode* merge(ListNode* p1,ListNode* p2){
        ListNode* dummy = new ListNode(-1);
        ListNode* p = dummy;
        while(p1 != NULL && p2 != NULL){
            if(p1->val < p2->val){
                p->next = p1;
                p1 = p1->next;
                p = p->next;
            }
            else{
                p->next = p2;
                p2 = p2->next;
                p = p->next;
            }
        }
        if(p1)
            p->next = p1;
        else
            p->next = p2;
        return dummy->next;
    }
};