Palindrome Linked List

题目：

Implement a function to check if a linked list is a palindrome.

分析：

本身逻辑不难，但是mock得时候reverseList居然写错了，写成了while(head != NULL), 然后死循环了。。。明明后面要取head->next->next的，怎么能取得出来呢！

正确写法是while(head->next != NULL)因为你实际上移动的是head->next这个节点呀，科科！

解法：

class Solution {
public:
    /**
     * @param head a ListNode
     * @return a boolean
     */

    ListNode* reverseList(ListNode* head) {
        ListNode dummy(0);
        dummy.next = head;
        while (head->next != NULL) {
            ListNode* next = head->next;
            head->next = next->next;
            next->next = dummy.next;
            dummy.next = next;
        }
        return dummy.next;
    }

    bool isPalindrome(ListNode* head) {
        // Write your code here
        if (head == NULL || head->next == NULL) {
            return true;
        }

        ListNode* slow = head; 
        ListNode* fast = head;

        while(fast->next != NULL && fast->next->next != NULL) {
            slow = slow->next;
            fast = fast->next->next;
        }

        ListNode* newHead = slow->next;
        slow->next = NULL;

        ListNode* head2 = reverseList(newHead);

        while (head != NULL && head2 != NULL) {
            if (head->val != head2->val) {
                return false;
            }
            head = head->next;
            head2 = head2->next;
        }

        return true;
    }
};