LeetCode-148. Sort List

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

        //找到中间结点
        ListNode pre = null, slow = head, fast = head;

        while (fast != null && fast.next != null) {
          pre = slow;
          slow = slow.next;
          fast = fast.next.next;
        }
        pre.next = null;

        //分治
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slow);

        //合并
        return merge(l1, l2);
      }

      ListNode merge(ListNode l1, ListNode l2) {
        ListNode l = new ListNode(0), p = l;

        while (l1 != null && l2 != null) {
          if (l1.val < l2.val) {
            p.next = l1;
            l1 = l1.next;
          } else {
            p.next = l2;
            l2 = l2.next;
          }
          p = p.next;
        }

        if (l1 != null)
          p.next = l1;

        if (l2 != null)
          p.next = l2;

        return l.next;
      }