leetcode 234. Palindrome Linked List

解题思路：
首先计算链表长度，然后将链表前半部分利用头插法逆序，与后半部分进行比较

原题目：

Given a singly linked list, determine if it is a palindrome.

AC解，C++代码，菜鸟一个，请大家多多指正

/** * 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) {            return true;        }        ListNode* p = head;        int length = 1;        while (p->next != NULL) {            p = p->next;            length++;        }        if (length == 1) {            return true;        }        int count = 2;        p = head->next;        head->next = NULL;        while (count <= (length / 2)) {            ListNode* tmp = p->next;            p->next = head;            head = p;            p = tmp;            count++;        }        if (length % 2 != 0) {            p = p->next;        }        ListNode* q = head;        count--;        while (count > 0) {            if (q->val != p->val) {                return false;            }            q = q->next;            p = p->next;            count--;        }        return true;    }};