C Exercises: Check if a singly linked list is palindrome or not

C Singly Linked List : Exercise-15 with Solution

Write a C program to check if a singly linked list is a palindrome or not.

Sample Solution:

C Code:

#include<stdio.h>
#include <stdlib.h>
#include <stdbool.h>

// Node structure for the linked list
struct Node {
    int data;
    struct Node* next;
};

// Function to create a new node
struct Node* newNode(int data) {
    struct Node* node = (struct Node*) malloc(sizeof(struct Node));
    node->data = data;
    node->next = NULL;
    return node;
}

// Function to reverse a linked list
struct Node* reverseList(struct Node* head) {
    struct Node* current = head;
    struct Node* next = NULL;
    struct Node* prev = NULL;

    while (current) {
        next = current->next;
        current->next = prev;
        prev = current;
        current = next;
    }

    return prev;
}

// Function to check if a linked list is a palindrome
bool isPalindrome(struct Node* head) {
    struct Node* slow = head;
    struct Node* fast = head;
    struct Node* prev_slow = head;
    struct Node* midnode = NULL;
    bool res = true;

    // Find the middle node
    while (fast && fast->next) {
        fast = fast->next->next;
        prev_slow = slow;
        slow = slow->next;
    }

    // If the number of nodes is odd, skip the middle node
    if (fast != NULL) {
        midnode = slow;
        slow = slow->next;
    }

    // Reverse the second half of the linked list
    slow = reverseList(slow);
    fast = head;

    // Compare the first and second half of the linked list
    while (slow != NULL) {
        if (fast->data != slow->data) {
            res = false;
            break;
        }

        fast = fast->next;
        slow = slow->next;
    }

    // Reverse the second half of the linked list again
    slow = reverseList(slow);

    // If the number of nodes is odd, set the next pointer of the middle node to NULL
    if (midnode != NULL) {
        prev_slow->next = midnode;
        midnode->next = slow;
    } else
        prev_slow->next = slow;

    return res;
}

// Function to print a linked list of integers
void displayList(struct Node* head) {
    while (head) {
        printf("%d ", head->data);
        head = head->next;
    }
    printf("\n");
}

// Function to print a linked list of characters
void displayList_char(struct Node* head) {
    while (head) {
        printf("%c", head->data);
        head = head->next;
    }
    printf("\n");
}

int main() {
    // Example 1: Integer linked list
	struct Node* head1 = newNode(1);
    head1->next = newNode(2);
    head1->next->next = newNode(3);
    head1->next->next->next = newNode(4);
    head1->next->next->next->next = newNode(5);
    printf("Original Singly List:\n");
	displayList(head1);
    if (isPalindrome(head1))
        printf("Linked list is a palindrome.\n");
    else
        printf("Linked list is not a palindrome.\n");

    // Example 2: Integer palindrome linked list
	struct Node* head2 = newNode(1);
    head2->next = newNode(2);
    head2->next->next = newNode(2);
    head2->next->next->next = newNode(1);
    printf("\nOriginal Singly List:\n");
	displayList(head2);
    if (isPalindrome(head2))
        printf("Linked list is a palindrome.\n");
    else
        printf("Linked list is not a palindrome.\n");

    // Example 3: Character palindrome linked list
	struct Node* head3 = newNode('M');
	head3->next = newNode('A');
    head3->next->next = newNode('D');
    head3->next->next->next = newNode('A');
    head3->next->next->next->next = newNode('M');
    printf("\nOriginal Singly List:\n");
	displayList_char(head3);
    if (isPalindrome(head3))
        printf("Linked list is a palindrome.\n");
    else
        printf("Linked list is not a palindrome.\n");        

    return 0;
}

Sample Output:

Original Singly List:
1 2 3 4 5
Linked list is not a palindrome.

Original Singly List:
1 2 2 1
Linked list is a palindrome.

Original Singly List:
MADAM
Linked list is a palindrome.

Flowchart :

C Programming Code Editor:

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