/**
 * 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* findMiddle(ListNode* head) {
        ListNode* fast = head;
        ListNode* slow = nullptr;

        while(fast != nullptr && fast->next != nullptr) {
            fast = fast->next->next;
            slow = (slow == nullptr) ? head : slow->next;
        }

        ListNode* mid = slow->next;
        slow->next = nullptr;
        return mid;
    }

    ListNode* merge(ListNode* left, ListNode* right) {
        ListNode* dummy = new ListNode(-1);
        ListNode* curr = dummy;

        while(left != nullptr && right != nullptr) {
            if(left->val < right->val) {
                curr->next = left;
                left = left->next;
            } else {
                curr->next = right;
                right = right->next;
            }
            curr = curr->next;
        }

        if(left != nullptr) {
            curr->next = left;
        } else {
            curr->next = right;
        }

        return dummy->next;
    }

    ListNode* sortList(ListNode* head) {
        if(head == nullptr) return nullptr;
        if(head->next == nullptr) return head;
        ListNode* mid = findMiddle(head);
        ListNode* left = sortList(head);
        ListNode* right = sortList(mid);
        return merge(left, right);
    }
};