LeetCode[234] Palindrome Linked List

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?
将链表后半部分反转，头尾个分配一个指针，依次向中间移动，若发现不同的值就不是回文
/**
* 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;
int count = 1;
ListNode* front = head, *back = head;
while (back->next != NULL)
{
count++;
back = back->next;
}
ListNode* p1 = head;
for (int i = 0; i < count / 2; i++)
p1 = p1->next;
ReverseList(p1);
for (int i = 0; i < count / 2; i++)
{
if (front->val != back->val)
return false;
front = front->next;
back = back->next;
}
return true;
}
void ReverseList(ListNode* head) {
ListNode* p1 = head, *p2 = head->next;
if (p2 == NULL)
return;
ListNode* p3 = p2->next;
p1->next = NULL;
while (p2 != NULL)
{
p2->next = p1;
p1 = p2, p2 = p3;
if (p2 != NULL)
p3 = p2->next;
}
}
};