【链表】Sort List（归并排序）

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<head2.val){
res=head1;
head1=head1.next;
}else{
res=head2;
head2=head2.next;
}
p=res;

while(head1!=null&&head2!=null){
if(head1.val<head2.val){
p.next=head1;
head1=head1.next;
}else{
p.next=head2;
head2=head2.next;
}
p=p.next;
}

if(head1!=null){
p.next=head1;
}else if(head2!=null){
p.next=head2;
}

return res;
}