问题

归并排序链表，常数级别空间复杂度

算法：快慢指针，归并排序，链表合并

经典题，关键点在于找到归并的中点。使用快慢指针，当快指针到头，满指针恰好就到中点了。

递归边界即单个节点或者空节点。

有序链表合并用三个指针搞定

时间复杂度

$O(NlogN)$

代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if(!head || !(head->next)) return head;
        ListNode *f = head->next, *s = head;
        while(f && f->next){
            f = f->next->next;
            s = s->next;
        }
        ListNode *t = s->next;
        s->next = nullptr;
        head = sortList(head);
        t = sortList(t);

        ListNode *dummy = new ListNode(-1);
        dummy->next = head;
        ListNode *p = dummy;
        while(head && t){
            if(head->val <= t->val){
                p->next = head;
                head = head->next;
            }else{
                p->next = t;
                t = t->next;
            }
            p = p->next;

        }
        p->next = head ? head : t;

        return dummy->next;
    }
};