Palindrome Linked-list

Question

Here's my code.
The concept is to traverse the whole linkedlist by taking two pointers fast and slow ,once slow is at the middle ,will reverse the list and compare it with the second half which is fast.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        ListNode *fast= head;
        ListNode *slow= head;
        
        while( fast!=NULL&&fast->next !=NULL)
            {
            fast=head->next->next;
            slow=head->next;
        }
        slow= reverse(slow);
        fast= head;
      
        while(slow!=NULL)
            { 
               if(fast->data!=slow->data)
                   return false
                   slow =slow->next;
            fast=fast->next;
            }
        return true;
    }
    public: ListNode reverse(ListNode *head){
       ListNode *prev = NULL;
        while(head!=NULL)
            {
            ListNode* nextnode=head->next;
            head->next=prev;
            prev=head;
            head=nextnode;
            
        }
        return prev;
    
    }
};

I am getting this minor error,plz help to rectify the code

Line 22: Char 15: error: assigning to 'ListNode *' from incompatible type 'ListNode'
slow= reverse(slow);

Answer 1

First of all as you can see below your reverse function returns object of ListNode type.

ListNode reverse(ListNode* head)
{
    ListNode* prev = NULL;
    while (head != NULL) {
        ListNode* nextnode = head->next;
        head->next = prev;
        prev = head;
        head = nextnode;
    }
    return prev;
}

However you are willing to return a "prev" which is of type ListNode*, so I suggest you to change the return type from "ListNode" to "ListNode*" in your reverse function like this:

ListNode* reverse(ListNode* head)