234. 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 || !head->next) return true;
        ListNode* slow(head), *fast(head);
        while(fast && fast->next){
            slow = slow->next;
            fast = fast->next->next;
        }
        if(fast) slow = slow->next; // if odd, move one step forward
        // Reverse second half
        ListNode* pre(NULL), *cur(slow), *next;
        while(cur){
            next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        // Compare first half with reversed second half
        while(pre){
            if(pre->val != head->val) return false;
            pre = pre->next;
            head = head->next;
        }
        return true;
    }
};