Sort List

这道题目关键是考虑怎么实现O nlogn ，所以要找到中点，迭代，其他和merge sort 2 list 一样

类似题目 merge k sorted list， merge 2 sorted list

参考 http://www.cnblogs.com/springfor/p/3869372.html

public class Solution {    // 问题关键要想到找中点，然后迭代产生O（nlogn）    public ListNode sortList(ListNode head) {        if(head==null || head.next==null) return head;                ListNode left = new ListNode(0);        ListNode right = new ListNode(0);        ListNode run = head;        ListNode wk = head;                while(run.next!=null && run.next.next!=null){            run = run.next.next;            wk= wk.next;        }        ListNode first = head;        ListNode second = wk.next;        wk.next = null;   // 干啥的？？                // 以下是想不到的，可以先排序一下        if(first!=second){            left = sortList(first);            right = sortList(second);        }                return mergeLists(left, right);            }        public ListNode mergeLists(ListNode left, ListNode right){        if(left==null) return right;        if(right==null) return left;        ListNode h = new ListNode(-1);        ListNode cur = h;                while(left!=null && right !=null){            if(left.val>right.val){                cur.next = right;                cur = cur.next;                right = right.next;            }else{                cur.next = left;                cur = cur.next;                left = left.next;            }                        }                if(left!=null) cur.next = left;        if(right!=null) cur.next = right;                return h.next;    }}