linkedlist_insertion.c

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

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

void linkedListTraversal(struct Node *ptr)
{
    while (ptr != NULL)
    {
        printf("Element: %d\n", ptr->data);
        ptr = ptr->next;
    }
}

// Case 1
// Create a struct Node* function insertAtFirst which will return the pointer to the new head.
// We’ll pass the current head pointer and the data to insert at the beginning, in the function.
// Create a new struct Node* pointer ptr, and assign it a new memory location in the heap.
// Assign head to the next member of the ptr structure using ptr-> next = head, and the given data to its data member.
// Return this pointer ptr.

struct Node * insertAtFirst(struct Node *head, int data){
    struct Node * ptr = (struct Node *) malloc(sizeof(struct Node));
    
    ptr->data = data;
    ptr->next = head;
    return ptr; 
}

// Case 2
// Create a struct Node* function insertAtIndex which will return the pointer to the head.
// We’ll pass the current head pointer and the data to insert and the index where it will get inserted, in the function.
// Create a new struct Node* pointer ptr, and assign it a new memory location in the heap.
// Create a new struct Node* pointer pointing to head, and run a loop until this pointer reaches the index, where we are inserting a new node.
// Assign p->next to the next member of the ptr structure using ptr-> next = p->next, and the given data to its data member.
// Break the connection between p and p->next by assigning p->next the new pointer. That is, p->next = ptr.
// Return head.

struct Node * insertAtIndex(struct Node *head, int data, int index){
    struct Node * ptr = (struct Node *) malloc(sizeof(struct Node));
    struct Node * p = head;
    int i = 0;

    while (i!=index-1)
    {
        p = p->next;
        i++;
    }

    ptr->data = data;
    ptr->next = p->next;
    p->next = ptr;
    return head;
}

// Case 3
// Inserting at the end is very similar to inserting at any index. The difference holds in the limit of the while loop. Here we run a loop until the pointer reaches the end and points to NULL.
// Assign NULL to the next member of the new ptr structure using ptr-> next = NULL, and the given data to its data member.
// Break the connection between p and NULL by assigning p->next the new pointer. That is, p->next = ptr.
// Return head.

struct Node * insertAtEnd(struct Node *head, int data){
    struct Node * ptr = (struct Node *) malloc(sizeof(struct Node));
    struct Node * p = head;

    ptr->data = data;

    while(p->next!=NULL){
        p = p->next;
    }
    
    p->next = ptr;
    ptr->next = NULL;
    return head;
}

// Case 4
// Here, we already have a struct Node* pointer to insert the new node just next to it.
// Create a struct Node* function insertAfterNode which will return the pointer to the head.
// Pass into this function, the head node, the previous node, and the data.
// Create a new struct Node* pointer ptr, and assign it a new memory location in the heap.
// Since we already have a struct Node* prevNode given as a parameter, use it as p we had in the previous functions.
// Assign prevNode->next to the next member of the ptr structure using ptr-> next = prevNode->next, and the given data to its data member.
// Break the connection between prevNode and prevNode->next by assigning prevNode->next the new pointer. That is, prevNode->next = ptr.
// Return head.

struct Node * insertAfterNode(struct Node *head, struct Node *prevNode, int data){
    struct Node * ptr = (struct Node *) malloc(sizeof(struct Node));

    ptr->data = data;
    ptr->next = prevNode->next;
    prevNode->next = ptr;

    
    return head;
}


int main(){
    struct Node *head;
    struct Node *second;
    struct Node *third;
    struct Node *fourth;

    // Allocate memory for nodes in the linked list in Heap
    head = (struct Node *)malloc(sizeof(struct Node));
    second = (struct Node *)malloc(sizeof(struct Node));
    third = (struct Node *)malloc(sizeof(struct Node));
    fourth = (struct Node *)malloc(sizeof(struct Node));

    // Link first and second nodes
    head->data = 7;
    head->next = second;

    // Link second and third nodes
    second->data = 11;
    second->next = third;

    // Link third and fourth nodes
    third->data = 41;
    third->next = fourth;

    // Terminate the list at the third node
    fourth->data = 66;
    fourth->next = NULL;

    printf("Linked list before insertion\n");
    linkedListTraversal(head);
    // head = insertAtFirst(head, 56);
    // head = insertAtIndex(head, 56, 1);
    // head = insertAtEnd(head, 56);
    head = insertAfterNode(head, third, 45);
    printf("\nLinked list after insertion\n");
    linkedListTraversal(head);

    
    return 0;
}