234 Palindrome Linked List

链接：https://leetcode.com/problems/palindrome-linked-list/
问题描述：
Given a singly linked list, determine if it is a palindrome.

Follow up:
Could you do it in O(n) time and O(1) space?

Hide Tags Linked List Two Pointers
Hide Similar Problems (E) Palindrome Number (E) Valid Palindrome (E) Reverse Linked List

检查一个链表是不是回文链表。我的思想是找到连标的中点，逆转链表的后一半，和前一半的链表做对比。还有可以用向量或者栈来做，不过这样浪费了空间。递归做法还不会。

class Solution {
public:
     ListNode* reverseList(ListNode* head)
    {

        ListNode *t=new ListNode(0),*cur=NULL;
        t->next=head;
        cur=head;
        head=head->next;
        cur->next=NULL;
        while(head)
        {
            t->next=head;
            head=head->next;
            t->next->next=cur;
            cur=t->next;

        }
        cur=t->next;
        delete t;
        return cur;
    };
    bool isPalindrome(ListNode* head) {
        if(head==NULL) return true;
        ListNode*p1=head,*p2=head;
        while(p1&&p1->next!=NULL)
        {
            p1=p1->next->next;
            p2=p2->next;
        }
        p2=reverseList(p2);
        p1=head;
        while(p2)
        {
            if(p2->val!=p1->val)
            return false;
            p1=p1->next;
            p2=p2->next;
        }
        return true;

    }
};