checking if a linked list is palindrome or not

Question

I am writing a program to check whether a singly linked list is a palindrome or not. I am using the concept of reversing through iteration.

I have inserted 2,3,5,3,2 in the linked list and reversed it based on the thought that if the linked list obtained after reversing is the same as before reversing then it is a palindrome. But I am not able to conclude the program with the match statement. How can i match both the list ?

Here is my code:

struct Node{
    int data;
    Node* next;
};
Node*head;

void Insert(int x)
{
    Node*temp=new Node();
    temp->data=x;
    temp->next=head;
    head=temp;  
}

void print()
{
    Node*temp=head;
    cout<<"list is";
    while(temp!=NULL) 
    {
        cout<<temp->data;
        temp=temp->next;
    }
}

void rec()
{
    Node*current, *prev, *next;
    current=head;
    prev=NULL;
    while(current!=NULL)
    {
        next= current->next;
        current->next= prev;
        prev= current;
        current=next;
    } 
    head=prev;
}

int main()
{
    clrscr();
    Node*head=NULL;
    Insert(2);
    Insert(3);
    Insert(5);
    Insert(3);
    Insert(2);
    cout<<"list before reversing is\n";
    print();

    cout<<"\n";
    cout<<"list after reversing is \n";
    rec();
    print();

    getch();
}

Spanky · Accepted Answer · 2015-06-03 12:23:53Z

1

instead of the separate function to reverse, have a function to for checking palindrome or not. In that function reverse the list and store in a temp list. Then iterate and compare each node->data. if all matches then its palindrome else you break out of loop and set false.

answered Jun 3, 2015 at 12:23

Spanky

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Comments

xryl669 · Accepted Answer · 2015-06-03 12:31:20Z

1

Make a double linked list (add a Node * prev) to your list's node. Then, it's a matter of iterating and comparing both iterator, like this:

bool isPalindrome(Node * head, Node * tail)
{
    Node * inc = head, * dec = tail;
    while (inc != nullptr && dec != nullptr)
    {
        if (inc->data != dec->data) return false;
        inc = inc->next;
        dec = dec->prev;
    }
    return inc == nullptr && dec == nullptr;
}

Always try to use the best container for the task.

answered Jun 3, 2015 at 12:31

xryl669

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sp2danny · Accepted Answer · 2015-06-03 13:02:26Z

0

Make it recursive, and check on the way back

#include <functional>

using namespace std;

struct Node
{
    int data;
    Node* next;
};

bool isPalin( Node* h )
{
    function<bool(Node*)> ip_rec = [&](Node* p) -> bool
    {
        if(p)
        {
            if( ! ip_rec(p->next) ) return false;
            if( p->data != h->data ) return false;
            h=h->next;
        }
        return true;
    };

    return ip_rec(h);
}

edited Jun 3, 2015 at 13:02

answered Jun 3, 2015 at 12:48

sp2danny

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Comments

FelixSFD · Accepted Answer · 2016-12-10 14:08:16Z

0

My solution in C++.....Very easy but not sure whether the code is optimized one...

bool isPalindrome(Node *head)
{

    int count=0;
    Node *ptr, *nptr;
    ptr=head;
    while(ptr!=NULL)
    {

        count++;
        ptr=ptr->next;
    }
    ptr=head;
    int arr[count],count1=0;
    while(ptr!=NULL)
    {
       // cout<<ptr->data;
        arr[count1]=ptr->data;
        count1++;
        ptr=ptr->next;

    }

    ptr=head;

 while(ptr!=NULL)
    {
        if(ptr->data!=arr[count-1])
        return false;
        ptr=ptr->next;
        count--;
        if(count==-1)
        return true;
    }

}

edited Dec 10, 2016 at 14:08

FelixSFD

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answered Dec 10, 2016 at 13:54

Skms

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