148. 排序链表

class Solution {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode p = head, q = head;
        //注意：要保证p在左半区的最后一个，如两个节点的左边一个。
        while (p.next != null && p.next.next != null) {
            p = p.next.next;
            q = q.next;
        }
        ListNode next = q.next;
        q.next = null;
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(next);
        return merge(l1, l2);
    }
    private ListNode merge(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;
        while (l1 != null && l2 != null) {
             if (l1.val < l2.val) {
                cur.next = l1;
                l1 = l1.next;
            } else {
                cur.next = l2;
                l2 = l2.next;
            }
             cur = cur.next;
        }
        cur.next = l1 == null ? l2 : l1;
        return dummy.next;
    }
}