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;

}

};