234. Palindrome Linked List

/** * 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 *p1 = head, *p2 = head;        while (p2->next && p2->next->next) {            p1 = p1->next;            p2 = p2->next->next;        }        // the second part's head        ListNode *secondHead = p1->next;                // reverse the first part        ListNode *firstHead = NULL;        bool evenNodes = p2->next != NULL;        p2 = head;        while (p2 != p1) {            ListNode *next = p2->next;            p2->next = firstHead;            firstHead = p2;            p2 = next;        }        if (evenNodes) {            p1->next = firstHead;            firstHead = p1;        }        while (firstHead && secondHead) {            if (firstHead->val != secondHead->val) {                return false;            }            firstHead = firstHead->next;            secondHead = secondHead->next;        }        return !firstHead && !secondHead;    }};