单链表排序

题目描述
给定一个无序单链表，实现单链表的排序(按升序排序)。

解法

    //解法1：归并排序
    // 时间O(NlogN) 空间O（logN)
    ListNode* sortInList(ListNode* head) {
        if (head == nullptr || head->next == nullptr) return head;
        //二分
        //注意：需要做到：a->b 能断开
        //ListNode *fast = head, *slow = head;
        ListNode *fast = head->next, *slow = head;
        while (fast && fast->next) {
            fast = fast->next->next;
            slow = slow->next;
        }
        ListNode* left = head;
        ListNode* right = slow->next;
        slow->next = nullptr;
        ListNode* sortLeft = sortInList(left);
        ListNode* sortRight = sortInList(right);

        //合
        ListNode* ans = merge(sortLeft, sortRight);
        return ans;
    }

    ListNode* merge(ListNode* l1, ListNode* l2) {
        if (l1 == nullptr || l2 == nullptr) return l1? l1:l2;
        ListNode dummy(0);
        ListNode* cur = &dummy;
        while (l1 && l2) {
            if (l1->val < l2->val) {
                cur->next = l1;
                cur = cur->next;
                l1 = l1->next;
            } else {
                cur->next = l2;
                cur = cur->next;
                l2 = l2->next;
            }
        }
        if (l1) cur->next = l1;
        if (l2) cur->next = l2;
        return dummy.next;
    }