题目：

Sort a linked list in O(n log n) time using constant space complexity.

思路：

nlogn的排序有快速排序、归并排序、堆排序。双向链表用快排比较适合，堆排序也可以用于链表，单向链表适合用归并排序。

/** * Definition for singly-linked list. * function ListNode(val) { *     this.val = val; *     this.next = null; * } *//** * @param {ListNode} head * @return {ListNode} */var sortList = function(head) {    if(head==null||head.next==null){        return head;    }else{        var slow=head,fast=head;        while(fast.next!=null&&fast.next.next!=null){            slow=slow.next;            fast=fast.next.next;        }        //拆成两个链表        fast=slow;        slow=slow.next;        fast.next=null;                fast=sortList(head);        slow=sortList(slow);        return merge(fast,slow);    }};function merge(head1,head2){    if(head1==null){        return head2;    }    if(head2==null){        return head1;    }    var res=new ListNode(),p=new ListNode();    if(head1.val