C - Insert a new node at the start of the Linked List

In this method, a new node is inserted at the beginning of the linked list. For example - if the given List is 10->20->30 and a new element 100 is added at the start, the Linked List becomes 100->10->20->30.

Inserting a new node at the beginning of the Linked List is very easy. First, a new node with given element is created. It is then added before the head of the given Linked List that makes the newly added node to new head of the Linked List by changing the head pointer to point to the new node.

The function push_front is created for this purpose. It is a 4-step process.

void push_front(struct Node** head_ref, int newElement) {
  
  //1. allocate a new node
  struct Node* newNode;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  
  //2. assign data element 
  newNode->data = newElement;
  
  //3. make next node of new node as head
  newNode->next = *head_ref;
  
  //4. make new node as head
  *head_ref = newNode;   
}

The below is a complete program that uses above discussed concept to insert new node at the start of the linked list.

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

//node structure
struct Node {
  int data;
  struct Node* next;
};

//Add new element at the start of the list
void push_front(struct Node** head_ref, int newElement) {
  struct Node* newNode;
  newNode = (struct Node*)malloc(sizeof(struct Node));  
  newNode->data = newElement;
  newNode->next = *head_ref; 
  *head_ref = newNode;   
}

//display the content of the list
void PrintList(struct Node* head_ref) {
  struct Node* temp = head_ref;
  if(head_ref != NULL) {
    printf("The list contains: ");
    while (temp != NULL) {
      printf("%i ",temp->data);
      temp = temp->next;  
    }
    printf("\n");
  } else {
    printf("The list is empty.\n");
  }   
}

// test the code  
int main() {
  struct Node* MyList = NULL;

  //Add three elements at the start of the list.
  push_front(&MyList, 10);
  push_front(&MyList, 20);
  push_front(&MyList, 30);
  PrintList(MyList);

  return 0; 
}

The above code will give the following output:

The list contains: 30 20 10