Leetcode Sort List

题目：Sort a linked list in O(n log n) time using constant space complexity.
答案，源代码：
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {

   public ListNode sortList(ListNode head) {

if(null==head){

return head;

}

ArrayList<ListNode> al = new ArrayList<ListNode>();

ListNode ln =head;

al.add(ln);

while (null != ln.next) {

ln=ln.next;

al.add(ln);

}

Collections.sort(al, new Comparator<ListNode>() {

@Override

public int compare(ListNode node1, ListNode node2) {

return node1.val-node2.val;

}});

ln=al.get(0);

ListNode node = ln;

for (int i = 1; i < al.size(); i++) {

node.next = new ListNode(0);

node.next.val=al.get(i).val;

node=node.next;

}

return ln;

    }

}