234. Palindrome Linked List

question:

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?

answer:

思路：把前半部分的链表翻转，再分别用两个指针从中间往两边遍历比较。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        ListNode *p,*q,*r;
        int i, l=0;
        p=q=head;
        r=NULL;
        if(head==NULL||head->next==NULL){
          return true;
        }
        while(p!=NULL){
            l++;
            p=p->next;
        }
        for(i=0;i<l/2;i++){
            p=q->next;
            q->next=r;
            r=q;
            q=p;
        }
        if(l%2==1){
            p=p->next;
        }
        while(p!=NULL&&r!=NULL){
            if(r->val!=p->val)
            return false;
            p=p->next;
            r=r->next;
        }
        return true;
    }
};