ListNode sortList(ListNode head) {
 if (head == null || head.next == null)
  return head;
 
 ListNode fast = head.next, slow = head;
 while (fast != null && fast.next != null) {
  fast = fast.next.next; // 一次移動兩位
  slow = slow.next; // 一次移動一位
 }
 
 ListNode tmp = slow.next; // 記錄后鏈表的頭節(jié)點
 slow.next = null; // 將前后鏈表斷開
 //...
}

ListNode sortList(ListNode head) {
 //...
 // left 為前鏈表的頭節(jié)點， right 為后鏈表的頭節(jié)點， h 為輔助節(jié)點
 while (left != null && right != null) {
  if (left.val < right.val) { 
   h.next = left;
   left = left.next;
  } else {
   h.next = right;
   right = right.next;
  }
  h = h.next;
 }
 h.next = left != null ? left : right;
 //...
}

ListNode sortList(ListNode head) {
    if (head == null || head.next ==null)
      return head;
    // cut過程
    ListNode fast = head.next, slow = head;
    while (fast != null && fast.next != null) {
      fast = fast.next.next;
      slow = slow.next;
    }
    ListNode tmp = slow.next;
    slow.next = null;
	// merage過程
    ListNode left = sortList(head);
    ListNode right = sortList(tmp);
    ListNode h = new ListNode(0);
    ListNode res = h;
    while (left != null && right != null) {
      if (left.val < right.val) {
        h.next = left;
        left = left.next;
      } else {
        h.next = right;
        right = right.next;
      }
      h = h.next;
    }
    h.next = left != null ? left : right;

    return res.next;
  }