Palindrome Linked List -- leetcode

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?

基本思路：

1. 先用快慢指針，找到鏈表的中間結點。

2. 將後半段進行逆轉

3. 將前半段和經過逆轉的後半段進行逐個比較。

此題不得不修改鏈表，這到底算不算O(1) space呢

/**
 * 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) return true;
        ListNode *walker = head;
        ListNode *runner = head->next;
        while (runner && runner->next) {
            walker = walker->next;
            runner = runner->next;
            runner = runner->next;
        }
        
        runner = walker->next;
        walker = NULL;
        while (runner) {
            auto bak = runner->next;
            runner->next = walker;
            walker = runner;
            runner = bak;
        }
        
        while (walker && walker->val == head->val) {
            walker = walker->next;
            head = head->next;
        }
        return !walker;
    }
};