Check linked list with a loop is palindrome or not

Given a linked list with a loop, the task is to find whether it is palindrome or not. You are not allowed to remove the loop.

Examples:

Input : 1 -> 2 -> 3 -> 2
             /|\      \|/
               ------- 1  
Output: Palindrome
Linked list is 1 2 3 2 1 which is a 
palindrome.

Input : 1 -> 2 -> 3 -> 4
             /|\      \|/
               ------- 1  
Output: Palindrome
Linked list is 1 2 3 4 1 which is a 
not palindrome.

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Algorithm:

Detect the loop using Floyd Cycle Detection Algorithm.
Then find the starting node of loop as discussed in this
Check linked list is palindrome or not as discussed in this

Below is the implementation.

C++

// C++ program to check if a linked list with 
// loop is palindrome or not. 
#include<bits/stdc++.h> 
using namespace std; 
  
/* Link list node */
struct Node 
{ 
    int data; 
    struct Node * next; 
}; 
  
/* Function to find loop starting node. 
loop_node --> Pointer to one of the loop nodes 
head --> Pointer to the start node of the linked list */
Node* getLoopstart(Node *loop_node, Node *head) 
{ 
    Node *ptr1 = loop_node; 
    Node *ptr2 = loop_node; 
  
    // Count the number of nodes in loop 
    unsigned int k = 1, i; 
    while (ptr1->next != ptr2) 
    { 
        ptr1 = ptr1->next; 
        k++; 
    } 
  
    // Fix one pointer to head 
    ptr1 = head; 
  
    // And the other pointer to k nodes after head 
    ptr2 = head; 
    for (i = 0; i < k; i++) 
        ptr2 = ptr2->next; 
  
    /* Move both pointers at the same pace, 
    they will meet at loop starting node */
    while (ptr2 != ptr1) 
    { 
        ptr1 = ptr1->next; 
        ptr2 = ptr2->next; 
    } 
    return ptr1; 
} 
  
/* This function detects and find loop starting 
  node  in the list*/
Node* detectAndgetLoopstarting(Node *head) 
{ 
    Node *slow_p = head, *fast_p = head,*loop_start; 
  
    //Start traversing list and detect loop 
    while (slow_p && fast_p && fast_p->next) 
    { 
        slow_p = slow_p->next; 
        fast_p = fast_p->next->next; 
  
        /* If slow_p and fast_p meet then find 
           the loop starting node*/
        if (slow_p == fast_p) 
        { 
            loop_start = getLoopstart(slow_p, head); 
            break; 
        } 
    } 
  
    // Return starting node of loop 
    return loop_start; 
} 
  
// Utility function to check if a linked list with loop 
// is palindrome with given starting point. 
bool isPalindromeUtil(Node *head, Node* loop_start) 
{ 
    Node *ptr = head; 
    stack<int> s; 
  
    // Traverse linked list until last node is equal 
    // to loop_start and store the elements till start 
    // in a stack 
    int count = 0; 
    while (ptr != loop_start || count != 1) 
    { 
        s.push(ptr->data); 
        if (ptr == loop_start) 
            count = 1; 
        ptr = ptr->next; 
    } 
    ptr = head; 
    count = 0; 
  
    // Traverse linked list until last node is 
    // equal to loop_start second time 
    while (ptr != loop_start || count != 1) 
    { 
        // Compare data of node with the top of stack 
        // If equal then continue 
        if (ptr->data == s.top()) 
            s.pop(); 
  
        // Else return false 
        else
            return false; 
  
        if (ptr == loop_start) 
            count = 1; 
        ptr = ptr->next; 
    } 
  
    // Return true if linked list is palindrome 
    return true; 
} 
  
// Function to find if linked list is palindrome or not 
bool isPalindrome(Node* head) 
{ 
    // Find the loop starting node 
    Node* loop_start = detectAndgetLoopstarting(head); 
  
    // Check if linked list is palindrome 
    return isPalindromeUtil(head, loop_start); 
} 
  
Node *newNode(int key) 
{ 
    Node *temp = new Node; 
    temp->data = key; 
    temp->next = NULL; 
    return temp; 
} 
  
/* Driver program to test above function*/
int main() 
{ 
    Node *head = newNode(50); 
    head->next = newNode(20); 
    head->next->next = newNode(15); 
    head->next->next->next = newNode(20); 
    head->next->next->next->next = newNode(50); 
  
    /* Create a loop for testing */
    head->next->next->next->next->next = head->next->next; 
  
    isPalindrome(head)? cout << "\nPalindrome"
                     : cout << "\nNot Palindrome"; 
  
    return 0; 
} 

Java

// Java program to check if a linked list 
// with loop is palindrome or not. 
import java.util.*; 
  
class GfG  
{ 
  
/* Link list node */
static class Node  
{  
    int data;  
    Node next;  
} 
  
/* Function to find loop starting node.  
loop_node --> Pointer to one of  
the loop nodes head --> Pointer to  
the start node of the linked list */
static Node getLoopstart(Node loop_node,  
                            Node head)  
{  
    Node ptr1 = loop_node;  
    Node ptr2 = loop_node;  
  
    // Count the number of nodes in loop  
    int k = 1, i;  
    while (ptr1.next != ptr2)  
    {  
        ptr1 = ptr1.next;  
        k++;  
    }  
  
    // Fix one pointer to head  
    ptr1 = head;  
  
    // And the other pointer to k  
    // nodes after head  
    ptr2 = head;  
    for (i = 0; i < k; i++)  
        ptr2 = ptr2.next;  
  
    /* Move both pointers at the same pace,  
    they will meet at loop starting node */
    while (ptr2 != ptr1)  
    {  
        ptr1 = ptr1.next;  
        ptr2 = ptr2.next;  
    }  
    return ptr1;  
}  
  
/* This function detects and find  
loop starting node in the list*/
static Node detectAndgetLoopstarting(Node head)  
{  
    Node slow_p = head, fast_p = head,loop_start = null;  
  
    //Start traversing list and detect loop  
    while (slow_p != null && fast_p != null &&  
            fast_p.next != null)  
    {  
        slow_p = slow_p.next;  
        fast_p = fast_p.next.next;  
  
        /* If slow_p and fast_p meet then find  
        the loop starting node*/
        if (slow_p == fast_p)  
        {  
            loop_start = getLoopstart(slow_p, head);  
            break;  
        }  
    }  
  
    // Return starting node of loop  
    return loop_start;  
}  
  
// Utility function to check if   
// a linked list with loop is  
// palindrome with given starting point.  
static boolean isPalindromeUtil(Node head, 
                            Node loop_start)  
{  
    Node ptr = head;  
    Stack<Integer> s = new Stack<Integer> ();  
  
    // Traverse linked list until last node   
    // is equal to loop_start and store the   
    // elements till start in a stack  
    int count = 0;  
    while (ptr != loop_start || count != 1)  
    {  
        s.push(ptr.data);  
        if (ptr == loop_start)  
            count = 1;  
        ptr = ptr.next;  
    }  
    ptr = head;  
    count = 0;  
  
    // Traverse linked list until last node is  
    // equal to loop_start second time  
    while (ptr != loop_start || count != 1)  
    {  
        // Compare data of node with the top of stack  
        // If equal then continue  
        if (ptr.data == s.peek())  
            s.pop();  
  
        // Else return false  
        else
            return false;  
  
        if (ptr == loop_start)  
            count = 1;  
        ptr = ptr.next;  
    }  
  
    // Return true if linked list is palindrome  
    return true;  
}  
  
// Function to find if linked list 
// is palindrome or not  
static boolean isPalindrome(Node head)  
{  
    // Find the loop starting node  
    Node loop_start = detectAndgetLoopstarting(head);  
  
    // Check if linked list is palindrome  
    return isPalindromeUtil(head, loop_start);  
}  
  
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)  
{  
    Node head = newNode(50);  
    head.next = newNode(20);  
    head.next.next = newNode(15);  
    head.next.next.next = newNode(20);  
    head.next.next.next.next = newNode(50);  
  
    /* Create a loop for testing */
    head.next.next.next.next.next = head.next.next;  
  
    if(isPalindrome(head) == true) 
        System.out.println("Palindrome"); 
    else
        System.out.println("Not Palindrome");  
  
} 
}  
  
// This code is contributed by prerna saini 

C#

// C# program to check if a linked list 
// with loop is palindrome or not. 
using System; 
using System.Collections.Generic;  
  
class GfG  
{ 
  
/* Link list node */
class Node  
{  
    public int data;  
    public Node next;  
} 
  
/* Function to find loop starting node.  
loop_node --> Pointer to one of  
the loop nodes head --> Pointer to  
the start node of the linked list */
static Node getLoopstart(Node loop_node,  
                            Node head)  
{  
    Node ptr1 = loop_node;  
    Node ptr2 = loop_node;  
  
    // Count the number of nodes in loop  
    int k = 1, i;  
    while (ptr1.next != ptr2)  
    {  
        ptr1 = ptr1.next;  
        k++;  
    }  
  
    // Fix one pointer to head  
    ptr1 = head;  
  
    // And the other pointer to k  
    // nodes after head  
    ptr2 = head;  
    for (i = 0; i < k; i++)  
        ptr2 = ptr2.next;  
  
    /* Move both pointers at the same pace,  
    they will meet at loop starting node */
    while (ptr2 != ptr1)  
    {  
        ptr1 = ptr1.next;  
        ptr2 = ptr2.next;  
    }  
    return ptr1;  
}  
  
/* This function detects and find  
loop starting node in the list*/
static Node detectAndgetLoopstarting(Node head)  
{  
    Node slow_p = head, fast_p = head,loop_start = null;  
  
    //Start traversing list and detect loop  
    while (slow_p != null && fast_p != null &&  
            fast_p.next != null)  
    {  
        slow_p = slow_p.next;  
        fast_p = fast_p.next.next;  
  
        /* If slow_p and fast_p meet then find  
        the loop starting node*/
        if (slow_p == fast_p)  
        {  
            loop_start = getLoopstart(slow_p, head);  
            break;  
        }  
    }  
  
    // Return starting node of loop  
    return loop_start;  
}  
  
// Utility function to check if  
// a linked list with loop is  
// palindrome with given starting point.  
static bool isPalindromeUtil(Node head, 
                            Node loop_start)  
{  
    Node ptr = head;  
    Stack<int> s = new Stack<int> ();  
  
    // Traverse linked list until last node  
    // is equal to loop_start and store the  
    // elements till start in a stack  
    int count = 0;  
    while (ptr != loop_start || count != 1)  
    {  
        s.Push(ptr.data);  
        if (ptr == loop_start)  
            count = 1;  
        ptr = ptr.next;  
    }  
    ptr = head;  
    count = 0;  
  
    // Traverse linked list until last node is  
    // equal to loop_start second time  
    while (ptr != loop_start || count != 1)  
    {  
        // Compare data of node with the top of stack  
        // If equal then continue  
        if (ptr.data == s.Peek())  
            s.Pop();  
  
        // Else return false  
        else
            return false;  
  
        if (ptr == loop_start)  
            count = 1;  
        ptr = ptr.next;  
    }  
  
    // Return true if linked list is palindrome  
    return true;  
}  
  
// Function to find if linked list 
// is palindrome or not  
static bool isPalindrome(Node head)  
{  
    // Find the loop starting node  
    Node loop_start = detectAndgetLoopstarting(head);  
  
    // Check if linked list is palindrome  
    return isPalindromeUtil(head, loop_start);  
}  
  
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)  
{  
    Node head = newNode(50);  
    head.next = newNode(20);  
    head.next.next = newNode(15);  
    head.next.next.next = newNode(20);  
    head.next.next.next.next = newNode(50);  
  
    /* Create a loop for testing */
    head.next.next.next.next.next = head.next.next;  
  
    if(isPalindrome(head) == true) 
        Console.WriteLine("Palindrome"); 
    else
        Console.WriteLine("Not Palindrome");  
} 
} 
  
/* This code is contributed by 29AjayKumar */

Output:

Palindrome

This article is contributed by Sahil Chhabra (akku). 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.

My Personal Notes

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

C#

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