Length of longest palindrome list in a linked list using O(1) extra space

Given a linked list, find length of the longest palindrome list that exist in that linked list.

Examples:

Input  : List = 2->3->7->3->2->12->24
Output : 5
The longest palindrome list is 2->3->7->3->2

Input  : List = 12->4->4->3->14
Output : 2
The longest palindrome list is 4->4



A simple solution could be to copy linked list content to array and then find longest palindromic subarray in array, but this solution is not allowed as it requires extra space.

The idea is based on iterative linked list reverse process. We iterate through given linked list and one by one reverse every prefix of linked list from left. After reversing a prefix, we find the longest common list beginning from reversed prefix and list after the reversed prefix.

Below is the implementation of above idea.

C++

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// C++ program to find longest palindrome
// sublist in a list in O(1) time.
#include<bits/stdc++.h>
using namespace std;
  
//structure of the linked list
struct Node
{
    int data;
    struct Node* next;
};
  
// function for counting the common elements
int countCommon(Node *a, Node *b)
{
    int count = 0;
  
    // loop to count coomon in the list starting
    // from node a and b
    for (; a && b; a = a->next, b = b->next)
  
        // increment the count for same values
        if (a->data == b->data)
            ++count;
        else
            break;
  
    return count;
}
  
// Returns length of the longest palindrome
// sublist in given list
int maxPalindrome(Node *head)
{
    int result = 0;
    Node *prev = NULL, *curr = head;
  
    // loop till the end of the linked list
    while (curr)
    {
        // The sublist from head to current
        // reversed.
        Node *next = curr->next;
        curr->next = prev;
  
        // check for odd length palindrome
        // by finding longest common list elements
        // beginning from prev and from next (We
        // exclude curr)
        result = max(result,
                     2*countCommon(prev, next)+1);
  
        // check for even length palindrome
        // by finding longest common list elements
        // beginning from curr and from next
        result = max(result,
                     2*countCommon(curr, next));
  
        // update prev and curr for next iteration
        prev = curr;
        curr = next;
    }
    return result;
}
  
// Utility function to create a new list node
Node *newNode(int key)
{
    Node *temp = new Node;
    temp->data = key;
    temp->next = NULL;
    return temp;
}
  
/* Driver program to test above functions*/
int main()
{
    /* Let us create a linked lists to test
       the functions
    Created list is a: 2->4->3->4->2->15 */
    Node *head = newNode(2);
    head->next = newNode(4);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(2);
    head->next->next->next->next->next = newNode(15);
  
    cout << maxPalindrome(head) << endl;
    return 0;
}

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Java














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// Java program to find longest palindrome  


// sublist in a list in O(1) time.  


class GfG  


{  


  


//structure of the linked list  


static class Node  


{  


    int data;  


    Node next;  


} 


  


// function for counting the common elements  


static int countCommon(Node a, Node b)  


{  


    int count = 0;  


  


    // loop to count coomon in the list starting  


    // from node a and b  


    for (; a != null && b != null; 


            a = a.next, b = b.next)  


  


        // increment the count for same values  


        if (a.data == b.data)  


            ++count;  


        else


            break;  


  


    return count;  


}  


  


// Returns length of the longest palindrome  


// sublist in given list  


static int maxPalindrome(Node head)  


{  


    int result = 0;  


    Node prev = null, curr = head;  


  


    // loop till the end of the linked list  


    while (curr != null)  


    {  


        // The sublist from head to current  


        // reversed.  


        Node next = curr.next;  


        curr.next = prev;  


  


        // check for odd length  


        // palindrome by finding  


        // longest common list elements  


        // beginning from prev and  


        // from next (We exclude curr)  


        result = Math.max(result,  


                    2 * countCommon(prev, next)+1);  


  


        // check for even length palindrome  


        // by finding longest common list elements  


        // beginning from curr and from next  


        result = Math.max(result,  


                    2*countCommon(curr, next));  


  


        // update prev and curr for next iteration  


        prev = curr;  


        curr = next;  


    }  


    return result;  


}  


  


// Utility function to create a new list node  


static Node newNode(int key)  


{  


    Node temp = new Node();  


    temp.data = key;  


    temp.next = null;  


    return temp;  


}  


  


/* Driver code*/


public static void main(String[] args)  


{  


    /* Let us create a linked lists to test  


    the functions  


    Created list is a: 2->4->3->4->2->15 */


    Node head = newNode(2);  


    head.next = newNode(4);  


    head.next.next = newNode(3);  


    head.next.next.next = newNode(4);  


    head.next.next.next.next = newNode(2);  


    head.next.next.next.next.next = newNode(15);  


  


    System.out.println(maxPalindrome(head));  


}  


}  


  


// This code is contributed by  


// Prerna Saini. 



















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Python














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# Python program to find longest palindrome  


# sublist in a list in O(1) time.  


  


# Linked List node  


class Node:  


    def __init__(self, data):  


        self.data = data  


        self.next = None


  


# function for counting the common elements  


def countCommon(a, b) : 


  


    count = 0


  


    # loop to count coomon in the list starting  


    # from node a and b  


    while ( a != None and b != None ) : 


  


        # increment the count for same values  


        if (a.data == b.data) : 


            count = count + 1


        else: 


            break


          


        a = a.next


        b = b.next


  


    return count  


  


# Returns length of the longest palindrome  


# sublist in given list  


def maxPalindrome(head) : 


  


    result = 0


    prev = None


    curr = head  


  


    # loop till the end of the linked list  


    while (curr != None) : 


      


        # The sublist from head to current  


        # reversed.  


        next = curr.next


        curr.next = prev  


  


        # check for odd length  


        # palindrome by finding  


        # longest common list elements  


        # beginning from prev and  


        # from next (We exclude curr)  


        result = max(result,  


                    2 * countCommon(prev, next) + 1)  


  


        # check for even length palindrome  


        # by finding longest common list elements  


        # beginning from curr and from next  


        result = max(result,  


                    2 * countCommon(curr, next))  


  


        # update prev and curr for next iteration  


        prev = curr  


        curr = next


      


    return result  


  


# Utility function to create a new list node  


def newNode(key) : 


  


    temp = Node(0)  


    temp.data = key  


    temp.next = None


    return temp  


  


# Driver code 


  


# Let us create a linked lists to test  


# the functions  


# Created list is a: 2->4->3->4->2->15  


head = newNode(2)  


head.next = newNode(4)  


head.next.next = newNode(3)  


head.next.next.next = newNode(4)  


head.next.next.next.next = newNode(2)  


head.next.next.next.next.next = newNode(15)  


  


print(maxPalindrome(head))  


  


# This code is contributed by Arnab Kundu 



















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C#














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// C# program to find longest palindrome  


// sublist in a list in O(1) time.  


using System; 


  


class GfG  


{  


  


//structure of the linked list  


public class Node  


{  


    public int data;  


    public Node next;  


}  


  


// function for counting the common elements  


static int countCommon(Node a, Node b)  


{  


    int count = 0;  


  


    // loop to count coomon in the list starting  


    // from node a and b  


    for (; a != null && b != null;  


            a = a.next, b = b.next)  


  


        // increment the count for same values  


        if (a.data == b.data)  


            ++count;  


        else


            break;  


  


    return count;  


}  


  


// Returns length of the longest palindrome  


// sublist in given list  


static int maxPalindrome(Node head)  


{  


    int result = 0;  


    Node prev = null, curr = head;  


  


    // loop till the end of the linked list  


    while (curr != null)  


    {  


        // The sublist from head to current  


        // reversed.  


        Node next = curr.next;  


        curr.next = prev;  


  


        // check for odd length  


        // palindrome by finding  


        // longest common list elements  


        // beginning from prev and  


        // from next (We exclude curr)  


        result = Math.Max(result,  


                    2 * countCommon(prev, next)+1);  


  


        // check for even length palindrome  


        // by finding longest common list elements  


        // beginning from curr and from next  


        result = Math.Max(result,  


                    2*countCommon(curr, next));  


  


        // update prev and curr for next iteration  


        prev = curr;  


        curr = next;  


    }  


    return result;  


}  


  


// Utility function to create a new list node  


static Node newNode(int key)  


{  


    Node temp = new Node();  


    temp.data = key;  


    temp.next = null;  


    return temp;  


}  


  


/* Driver code*/


public static void Main(String []args)  


{  


    /* Let us create a linked lists to test  


    the functions  


    Created list is a: 2->4->3->4->2->15 */


    Node head = newNode(2);  


    head.next = newNode(4);  


    head.next.next = newNode(3);  


    head.next.next.next = newNode(4);  


    head.next.next.next.next = newNode(2);  


    head.next.next.next.next.next = newNode(15);  


  


    Console.WriteLine(maxPalindrome(head));  


}  


}  


  


// This code is contributed by Arnab Kundu 



















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Output : 


5


Time Complexity : O(n2)


Note that the above code modifies the given linked list and may not work if modifications to linked list are not allowed. However we can finally do one more reverse to get original list back.


This article is contributed by Niteesh kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.


Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.





    

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