/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
        // base case: 长度为[0-1]
        if (head == null || head.next == null) { return head; }
        // slow停在下位中位点，是第二部分的开头
        // pre是slow的前一个节点，是第一部分的结尾
        ListNode slow = head, fast = head, pre = null;
        while (fast != null && fast.next != null) {
            pre = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        pre.next = null;
        ListNode left = sortList(head);
        ListNode right = sortList(slow);
        return merge(left,right);
    }
    public ListNode merge(ListNode list1, ListNode list2) {
        ListNode dummy = new ListNode(0), cur = dummy, cur1 = list1, cur2 = list2;
        while (cur1 != null && cur2 != null) {
            if (cur1.val <= cur2.val) {
                cur.next = cur1;
                cur1 = cur1.next;
            } else {
                cur.next = cur2;
                cur2 = cur2.next;
            }
            cur = cur.next;
        }
        if (cur1 != null) { cur.next = cur1; }
        if (cur2 != null) { cur.next = cur2; }
        return dummy.next;
    }
}