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class Solution {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }

        ListNode mid = splitMid(head);

        ListNode left = sortList(head);
        ListNode right = sortList(mid);

        return merge(left, right);
    }

    private ListNode splitMid(ListNode head) {
        ListNode slow = head, fast = head, prev = null;
        while (fast != null && fast.next != null) {
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        prev.next = null;
        return slow;
    }

    private ListNode merge(ListNode a, ListNode b) {
        ListNode dummy = new ListNode(0), p = dummy;
        while (a != null && b != null) {
            if (a.val <= b.val) {
                p.next = a;
                a = a.next;
            } else {
                p.next = b;
                b = b.next;
            }
            p = p.next;
        }
        p.next = (a != null) ? a : b;
        return dummy.next;
    }
}