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}.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
struct ListNode
{
	int val;
	ListNode *next;
	ListNode(int x) : val(x), next(NULL) {}
};
class Solution {
public:
    void reorderList(ListNode *head) {
        if(head == NULL || head->next == NULL) return ;
		ListNode *fast = head->next;
		ListNode *slow = head;
		//找到链表的中间结点
		while (fast != NULL && fast->next != NULL)
		{
			fast = fast->next->next;
			slow = slow->next;
		}
		ListNode *p = slow->next;
		slow->next = NULL;//将链表分成两部分
		ListNode *q = NULL;
		while (p != NULL)//逆序后半部分的链表
		{
			ListNode *tmp = p->next;
			p->next = q;
			q = p;
			p = tmp;
		}
		ListNode *p1 = head;
		ListNode *p2 = q;
		while (p1 != NULL && p2 != NULL)//合并两部分链表
		{
			ListNode *t1 = p1->next;
			ListNode *t2 = p2->next;
			p1->next = p2;
			p2->next = t1;
			p1 = t1;
			p2 = t2;
		}
    }
};