链表排序（归并排序）

归并排序：

#include<iostream>

using namespace std;
struct ListNode{
	int val;
	ListNode *next;
	ListNode(int x):val(x),next(NULL){}
};
class Solution{
public:
	ListNode*sortList(ListNode *head)
	{
		if(!head||!head->next) return head;
		ListNode *p=head,*q=head->next;
		while(q&&q->next)
		{
			p=p->next;
			q=q->next->next;
		
		}
		ListNode*left =sortList(p->next);
		p->next=NULL;
		ListNode*right =sortList(head);
		return merge(left,right);
	
	}
	ListNode *merge(ListNode *left,ListNode *right)
	{
		ListNode dummy(0);
		ListNode *p=&dummy;
		while(left&&right)
		{
			if(left->val<right->val)
			{
				p->next=left;
				left=left->next;
			}
			else
			{
				p->next=right;
				right=right->next;
			}
			p=p->next;
		
		}
		if(left) p->next=left;
		if(right)p->next=right;
		return dummy.next;
	
	}

};