[链表与树]排序链表

在 O(n log n) 时间复杂度和常数级空间复杂度下，对链表进行排序。

示例 1:

输入: 4->2->1->3
输出: 1->2->3->4
示例 2:

输入: -1->5->3->4->0
输出: -1->0->3->4->5

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        if( head == null ){
                return head;
            }
            
            List<ListNode> list = new LinkedList<ListNode>();
            
            ListNode node = head;
            while( node!=null ){
               list.add(node);
               node = node.next;
            }
       
            
            list.sort(new Comparator<ListNode>() {
                @Override
                public int compare(ListNode o1, ListNode o2) {
                    return o1.val > o2.val ? 1 : (o1.val == o2.val ? 0 : -1);
                }
            });
        head = new ListNode(0);
        ListNode r = head;
        for (ListNode n : list) {
            r.next = n;
            r = n;
        }
        r.next = null;           
        return head.next;
    }
}