Sort List — C++

// Time:  O(nlogn)
// Space: O(logn)

/**
 * 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;
        }

        auto slow = head, fast = head;
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
        }

        // Split linked list.
        fast = slow;
        slow = slow->next;
        fast->next = nullptr;

        return mergeTwoLists(sortList(head), sortList(slow));
    }

private:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode dummy{0};
        auto curr = &dummy;

        while (l1 && l2) {
            if (l1->val <= l2->val) {
                curr->next = l1;
                l1 = l1->next;
            } else {
                curr->next = l2;
                l2 = l2->next;
            }
            curr = curr->next;
        }
        curr->next = l1 ? l1 : l2;

        return dummy.next;
    }
};