Palindrome Linked List

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param {ListNode} head
    # @return {boolean}
    def isPalindrome(self, head):
        if head is None:
            return True
        #find mid node
        fast = slow = head
        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next
        #reverse second half
        p, last = slow.next, None
        while p:
            next = p.next
            p.next = last
            last, p = p, next
        #check palindrome
        p1, p2 = last, head
        while p1 and p1.val == p2.val:
            p1, p2 = p1.next, p2.next
        #resume linked list(optional)
        p, last = last, None
        while p:
            next = p.next
            p.next = last
            last, p = p, next
        slow.next = last
        return p1 is None

class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if (!head || !head->next) return true;
        ListNode *slow = head, *fast = head;
        stack<int> s;
        s.push(head->val);
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
            s.push(slow->val);
        }
        if (!fast->next) s.pop();
        while (slow->next) {
            slow = slow->next;
            int tmp = s.top(); s.pop();
            if (tmp != slow->val) return false;
        }
        return true;
    }
};

class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if (!head || !head->next) return true;
        ListNode *slow = head, *fast = head;
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode *last = slow->next, *pre = head;
        while (last->next) {
            ListNode *tmp = last->next;
            last->next = tmp->next;
            tmp->next = slow->next;
            slow->next = tmp;
        }
        while (slow->next) {
            slow = slow->next;
            if (pre->val != slow->val) return false;
            pre = pre->next;
        }
        return true;
    }
};

public boolean isPalindrome(ListNode head) {
    ListNode fast = head, slow = head;
    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
    }
    if (fast != null) { // odd nodes: let right half smaller
        slow = slow.next;
    }
    slow = reverse(slow);
    fast = head;
    
    while (slow != null) {
        if (fast.val != slow.val) {
            return false;
        }
        fast = fast.next;
        slow = slow.next;
    }
    return true;
}

public ListNode reverse(ListNode head) {
    ListNode prev = null;
    while (head != null) {
        ListNode next = head.next;
        head.next = prev;
        prev = head;
        head = next;
    }
    return prev;
}