C Program to Reverse a Linked List

Reversing a linked list means re-arranging the next (connection) pointers. Head will point to the last node and the first node will point to NULL. Direction of other next pointers will change.

If a linked list looks like this:

After reversal, it will look like:

Logic to Reverse a Linked List

1. Take the first node out of the linked list. One new pointer will point to the first node and the head will move to the next node.
2. Insert the node at the from of the new list.
3. Repeat previous two steps until all nodes move to the new list.
4. Pointer the head of the original linked list to the first node of the new list.

Reverse Linked List Program

/*
 * File name: test.c
 * Compile: cc test.c
 * */
#include <stdio.h>
#include <stdlib.h>

struct node{
    int val;
    struct node *next;
};

void print_list(struct node *head)
{
    printf("H->");

    while(head)
    {
        printf("%d->", head->val);
        head = head->next;
    }

    printf("|||\n\n");
}

void insert_front(struct node **head, int value)
{
    struct node * new_node = NULL;

    new_node = (struct node *)malloc(sizeof(struct node));

    if (new_node == NULL)
    {
        printf("Failed to insert element. Out of memory");
    }

    new_node->val = value;
    new_node->next = *head;

    *head = new_node;
}

void reverse_linked_list(struct node **head) {
  struct node *new_head = NULL; /*head of the reversed list*/
  struct node *tmp = NULL;

  while(*head) {
    tmp = *head; /*tmp points the first node of the current list*/
    *head = (*head)->next;

    /*Insert tmp at the front of the reversed list.*/
    tmp->next = new_head;
    new_head = tmp;
  }

  *head = new_head;
}

void main()
{
    int count = 0, i, val;
    struct node * head = NULL;

    printf("Enter number of elements: ");
    scanf("%d", &count);

    for (i = 0; i < count; i++)
    {
        printf("Enter %dth element: ", i);
        scanf("%d", &val);
        insert_front(&head, val);
    }

    printf("Initial List: ");
    print_list(head);

    reverse_linked_list(&head);

    printf("Reversed List: ");
    print_list(head);
}

This program first takes input of integers and put them into a linked list using insert_front() function. Then the reverse_linked_list() is called to reverse the list.

Here is the output of the program.

Using Three Pointers

Instead of constructing a new list, we can reverse a linked list with the help of three pointers.

Logic to Reverse a Linked List Using There Pointer.

1. Initialize three pointers, prev to NULL, cur to head and next to head->next.
2. Until cur becomes NULL, assign next to next->next, cur->next to prev, prev to cur and cur to next.
3. At the end point head to prev.

Here is the modified reverse_linked_list() function.

void reverse_linked_list(struct node **head) {
  struct node *prev = NULL;
  struct node *cur = NULL;
  struct node *next = NULL;

  cur = *head;
  if(*head) next = (*head)->next;

  while(cur) {
    next = next->next;
    cur->next = prev;
    prev = cur;
    cur = next;
  }
  *head = prev;
}

Read also: Sort a Linked List.

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