Given a singly linked list, we have to convert it into circular linked list. For example, we have been given a singly linked list with four nodes and we want to convert this singly linked list into circular linked list.

The above singly linked list is converted into circular linked list.

**Approach:** The idea is to traverse the singly linked list and check if the node is the last node or not. If the node is the last node i.e pointing to NULL then make it point to the starting node i.e head node. Below is the implementation of this approach.

// Program for converting singly linked list // into circular linked list. #include <bits/stdc++.h> /* Linked list node */ struct Node { int data; struct Node* next; }; // Function that convert singly linked list // into circular linked list. struct Node* circular(struct Node* head) { // declare a node variable start and // assign head node into start node. struct Node* start = head; // check that while head->next not equal // to NULL then head points to next node. while (head->next != NULL) head = head->next; // if head->next points to NULL then // start assign to the head->next node. head->next = start; return start; } void push(struct Node** head, int data) { // Allocate dynamic memory for newNode. struct Node* newNode = (struct Node*)malloc (sizeof(struct Node)); // Assign the data into newNode. newNode->data = data; // newNode->next assign the address of // head node. newNode->next = (*head); // newNode become the headNode. (*head) = newNode; } // Function that display the elements of // circular linked list. void displayList(struct Node* node) { struct Node* start = node; while (node->next != start) { printf("%d ", node->data); node = node->next; } // Display the last node of circular // linked list. printf("%d ", node->data); } // Drier program to test the functions int main() { // Start with empty list struct Node* head = NULL; // Using push() function to construct // singly linked list // 17->22->13->14->15 push(&head, 15); push(&head, 14); push(&head, 13); push(&head, 22); push(&head, 17); // Call the circular_list function that // convert singly linked list to circular // linked list. circular(head); printf("Display list: \n"); displayList(head); return 0; }

Output:

Display list: 17 22 13 14 15

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