Problem

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?

Example

Input: 1->2
Output: false

Input: 1->2->2->1
Output: true

Code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
static int var = [](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if(head==NULL)
            return true;
        ListNode *fast=head;
        ListNode *slow=head;
        ListNode *reverse=NULL;  
        while(fast->next!=NULL && fast->next->next!=NULL){
            fast = fast->next->next;
            ListNode *temp = slow;
            slow = slow->next;
            temp->next = reverse;
            reverse = temp;
        }
        if(fast->next==NULL){
            slow = slow->next;
        }else{
            ListNode *temp = slow;
            slow = slow->next;
            temp->next = reverse;
            reverse = temp; 
        }
        while(slow){
            if(slow->val!=reverse->val)
                return false;
            slow=slow->next;
            reverse=reverse->next;
        }
        return true;
    }
};

Result

234. Palindrome Linked List.png