DATA STRUCTURES PROGRAM • SINGLY LINKED LISTS

Insert a Node at the Beginning of a Singly Linked List

Learn how to insert a node at the beginning of a singly linked list using a clear C program.

BeginnerLinked listNodePointers

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

10 20 30

COMPLETE C PROGRAM

Complete C implementation

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

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

struct Node *create_node(int value)
{
    struct Node *node = malloc(sizeof *node);
    if (node == NULL) { perror("malloc"); exit(EXIT_FAILURE); }
    node->data = value;
    node->next = NULL;
    return node;
}

void append(struct Node **head, int value)
{
    struct Node *node = create_node(value);
    if (*head == NULL) { *head = node; return; }
    struct Node *current = *head;
    while (current->next != NULL) current = current->next;
    current->next = node;
}

void print_list(const struct Node *head)
{
    while (head != NULL) { printf("%d ", head->data); head = head->next; }
    putchar('\n');
}

void free_list(struct Node *head)
{
    while (head != NULL) { struct Node *next = head->next; free(head); head = next; }
}

int main(void)
{
    struct Node *head = NULL;
    append(&head, 20); append(&head, 30);
    struct Node *node = create_node(10);
    node->next = head;
    head = node;
    print_list(head);
    free_list(head);
    return 0;
}

CURRENT STEP

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SELECTED LINE

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EXPECTED OUTPUT FOR THE SAMPLE

10 20 30

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Point the new node at the old head and replace head with the new node.
Display the computed result.

Point the new node at the old head and replace head with the new node.

EFFICIENCY

Time and space complexity

Time complexity

O(1)

Auxiliary space

O(n)

DEBUGGING CHECKLIST

Common mistakes

Check this

Use the correct format specifier for every variable.

Check this

Initialize variables before using their values.

Check this

Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.