void reorderList(ListNode* head){

if (head ==NULL || head->next == NULL || head->next->next == NULL)

return;

//快慢指针找中点

ListNode* fast = head;

ListNode* low = head;

while (fast->next != NULL && fast->next->next != NULL)

{

fast = fast->next->next;

low = low->next;

}

//对low后面的部分逆序

fast = low->next;

low->next = NULL;

while (fast != NULL)

{

ListNode* temp = fast->next;

fast->next = low->next;

low->next = fast;

fast = temp;

}

//合并low前面和后面两部分

ListNode* p = head;

ListNode* q = low->next;

while (p != NULL && q != NULL)

{

low->next = q->next;

q->next = p->next;

p->next = q;

p = q->next;

q = low->next;

}

}