判断一个链表是否为回文结构

给定一个链表，请判断该链表是否为回文结构。

struct ListNode {
	int val;
 	struct ListNode *next;
};

bool isPail(struct ListNode* head ) {
    struct ListNode* p1 = head;//快指针
    struct ListNode* p2 = head;//慢指针
    struct ListNode* n = NULL;
    if(NULL == head || NULL == head->next)    return true;
    while(NULL!=p1 && NULL!=p1->next)
    {
        p1 = p1->next->next;
        p2 = p2->next;
    }
    p1 = p2->next;
    p2->next = NULL;
    while(NULL != p1)
    {
        n = p1->next;
        p1->next = p2;
        p2 = p1;
        p1 = n;
    }
    p1 = head;
    while(NULL!=p1 && NULL!=p2)
    {
        if(p1->val != p2->val)    return false;
        p1 = p1->next;
        p2 = p2->next;
    }
    return true;
}