143.Reorder List

143.Reorder List  Given a singly linked list L: L0→L1→…→Ln-1→Ln, reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→… You must do this in-place without altering the nodes' values. For example, Given {1,2,3,4}, reorder it to {1,4,2,3}.
https://leetcode.com/problems/reorder-list/description

/*    
143.Reorder List 
                  ，         。              ，
     O(n)
*/
class Solution{
pubilic:
	void recorderList(ListNode* head){
		if(!head || !head->next) return;
		ListNode *slow = head, *fast = head, *p=head, *q=head;
		while(fast->next && fast->next->next)
			slow = slow->next,slow->next = NULL;
		fast = slow->next,slow->next = NULL;
		p = fast, q = fast->next, fast->next = NULL;
		while(q)
		{
			auto tem = q->next;
			q->next = p;
			p = q, q = tem;
		}
		q = head;
		while(q&&p)
		{
			auto tem1 = q->next, tem2 = p->next;
			p->next = q->next;
			q->next = p;
			q = tem1, p = tem2;
		}
	}
}