6. Circular Linked List

6.1 Structure

In a circular linked list, the last node points back to the first node (or head).

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

Visual Representation:

      ┌───────────────────────────┐
      ↓                           │
HEAD → [10|●] → [20|●] → [30|●] ──┘

6.2 Operations

6.2.1 Insert at Beginning

void insertAtBeginning(struct Node **head, int value) {
    struct Node *newNode = (struct Node*)malloc(sizeof(struct Node));
    if (newNode == NULL) {
        printf("Memory allocation failed!\n");
        return;
    }
    
    newNode->data = value;
    
    // If list is empty
    if (*head == NULL) {
        newNode->next = newNode;  // Points to itself
        *head = newNode;
        return;
    }
    
    // Find last node
    struct Node *temp = *head;
    while (temp->next != *head) {
        temp = temp->next;
    }
    
    // Insert at beginning
    newNode->next = *head;
    temp->next = newNode;
    *head = newNode;
}

6.2.2 Display Circular List

void displayCircular(struct Node *head) {
    if (head == NULL) {
        printf("List is empty!\n");
        return;
    }
    
    struct Node *temp = head;
    printf("List: ");
    
    do {
        printf("%d", temp->data);
        temp = temp->next;
        if (temp != head) {
            printf(" → ");
        }
    } while (temp != head);
    
    printf(" → (back to %d)\n", head->data);
}

6.2.3 Delete Node

void deleteNode(struct Node **head, int value) {
    if (*head == NULL) {
        printf("List is empty!\n");
        return;
    }
    
    struct Node *temp = *head;
    struct Node *prev = NULL;
    
    // If head node contains the value
    if (temp->data == value) {
        // If only one node
        if (temp->next == *head) {
            free(temp);
            *head = NULL;
            return;
        }
        
        // Find last node
        while (temp->next != *head) {
            temp = temp->next;
        }
        
        // Delete head
        temp->next = (*head)->next;
        free(*head);
        *head = temp->next;
        return;
    }
    
    // Search for the node
    prev = *head;
    temp = (*head)->next;
    
    while (temp != *head && temp->data != value) {
        prev = temp;
        temp = temp->next;
    }
    
    // If value not found
    if (temp == *head) {
        printf("Value %d not found!\n", value);
        return;
    }
    
    // Delete node
    prev->next = temp->next;
    free(temp);
}

6.2.4 Count Nodes

int countNodes(struct Node *head) {
    if (head == NULL) {
        return 0;
    }
    
    int count = 1;
    struct Node *temp = head->next;
    
    while (temp != head) {
        count++;
        temp = temp->next;
    }
    
    return count;
}