单链表的归并排序：时间复杂度O(nlogn),空间复杂度O(1)

/**
* Definition for singly-linked list.
* 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*pslow=head;
ListNode*pfast=head->next;
while(pfast&&pfast->next){
pslow=pslow->next;
pfast=pfast->next->next;
}
ListNode*h2=pslow->next;
pslow->next=NULL;
ListNode*p1=sortList(head);
ListNode*p2=sortList(h2);
return mergeList(p1,p2);

}

ListNode* mergeList(ListNode* headA,ListNode*headB){
ListNode*newhead=new ListNode(0);
ListNode*tail=newhead;
while(headA&&headB){
if(headA->val<headB->val){
tail->next=headA;
tail=headA;
headA=headA->next;
}else{
tail->next=headB;
tail=headB;
headB=headB->next;
}
}
while(headA) {
tail->next=headA;
tail=headA;
headA=headA->next;
}
while(headB) {
tail->next=headB;
tail=headB;
headB=headB->next;
}
ListNode* rehead=newhead->next;
delete newhead;
return rehead;
}

};