234. Palindrome Linked List（判断链表是不是回文的）

234. Palindrome Linked List

 

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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?

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思路：快慢指针，将前半链表翻转，然后与后面链表比较

/**
 * 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 *slow=head,*fast=head,*rev=NULL,*temp;
        while(fast&&fast->next)
        {
            fast=fast->next->next;
            temp=slow->next;
            slow->next=rev;
            rev=slow;
            slow=temp;
        }
        if(fast) slow=slow->next;
        while(slow)
        {
            if(slow->val!=rev->val) return false;
            slow=slow->next;
            rev=rev->next;
        }
        return rev==NULL;
    }
};