Leetcode 234. Palindrome Linked List (Easy) (cpp)

/*

234. Palindrome Linked List (Easy)

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

*/
/**
* 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 || head -> next == NULL) {
return true;
}
ListNode *mid = head;
ListNode *last = head;
while (last -> next != NULL && last -> next -> next != NULL) {
mid = mid -> next;
last = last -> next -> next;
}
mid -> next = reverse(mid -> next);
mid = mid -> next;
while (mid != NULL) {
if (mid -> val != head -> val) {
return false;
}
mid = mid -> next;
head = head -> next;
}
return true;
}
ListNode* reverse(ListNode* head) {
ListNode *prev = NULL;
ListNode *next = NULL;
while (head != NULL) {
next = head -> next;
head -> next = prev;
prev = head;
head = next;
}
return prev;
}
};