链表归并排序

class Solution {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode slow = head, fast = head, prev = head;
        while (fast != null && fast.next != null) {
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        prev.next = null;
        ListNode list1 = sortList(head);
        ListNode list2 = sortList(slow);
        return merge(list1, list2);
    }

    public ListNode merge(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode(-1), curr = dummy;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                curr.next = list1;
                list1 = list1.next;
            } else {
                curr.next = list2;
                list2 = list2.next;
            }
            curr = curr.next;
        }
        if (list1 != null) {
            curr.next = list1;
        }
        if (list2 != null) {
            curr.next = list2;
        }
        return dummy.next;
    }
}