Palindrome Linked List.cpp

/**
 * 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) {
        if(head == NULL || head->next == NULL){
            return true;
        }
        ListNode* slow = head;
        ListNode* fast = head;
        while(fast->next && fast->next->next){
            slow = slow->next;
            fast = fast->next->next;
        }
        slow = reverseList(slow->next);
        while(slow){
            if(head->val != slow->val){
                return false;
            }
            head = head->next;
            slow = slow->next;
        }
        return true;
    }
    ListNode* reverseList(ListNode* head){
        ListNode* pre = NULL;
        ListNode* next = NULL;
        while(head!=NULL){
            next=head->next;
            head->next=pre;
            pre=head;
            head=next;
        }
        return pre;
    }
};