How to Remove Duplicate Entries from Linked List in C Programming?

Here we’ll see how to write C program to remove duplicate entries from linked list. Duplicate entry means nodes with same value appear multiple times in the list.

Linked list in the diagram above contains duplicate entries, 43, 24 and 5 appeared twice. We’ll write a program to remove these duplicate elements such that they appear only once in the result linked list along with the non-duplicate elements. Another import thing, the order of the elements will remain intact in the result linked list. The result will look like the diagram below

C Program to Remove Duplicate Entries from Linked List

#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");
}

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 delete_node(struct node **head, struct node *node)
{
 struct node *tmp = NULL;

 if (head == NULL || *head == NULL) return;

 if(*head == node)
 {
  /*Delete the head node*/
  *head = (*head)->next;
  free(node);
  return;
 }

 tmp = *head;

 while(tmp->next && tmp->next != node) tmp = tmp->next;

 if(tmp->next)
 {
  tmp->next = tmp->next->next;
  free(node);
 }
 else
 {
  printf("Node not found in the list.\n");
 }
}

void remove_duplicate(struct node *head)
{
 struct node *tmp = NULL;
 struct node *tmp1 = NULL;
 struct node *tmp2 = NULL;

 if (head == NULL) return;

 tmp = head;
 while(tmp)
 {
  tmp1 = tmp->next;

  while(tmp1)
  {
    tmp2 = tmp1;
    tmp1 = tmp1->next;

    if(tmp->val == tmp2->val) delete_node(&head, tmp2);
  }

  tmp = tmp->next;
 }
}

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);

 remove_duplicate(head);

 printf("List after removing duplicate entries: ");
 print_list(head);
}

Output of the Program

If you run the program the output would look like

Enter number of elements: 8
Enter 1th element: 43
Enter 2th element: 24
Enter 4th element: 5
Enter 5th element: 5
Enter 6th element: 24
Enter 7th element: 65
Enter 8th element: 43
Enter 9th element: 6
Initial List: H->6->43->65->24->5->5->24->43->|||
List after removing duplicate entries: H->6->43->65->24->5->|||

Explanation of the Program

The program takes input from user to construct the linked list. The user needs to enter some duplicate entries to see the affect. The remove_duplicate() function is used to remove duplicate entries from the linked list passed as argument.

void remove_duplicate(struct node *head)
{
 struct node *tmp = NULL;
 struct node *tmp1 = NULL;
 struct node *tmp2 = NULL;

 if (head == NULL) return;

 tmp = head;
 while(tmp)
 {
  tmp1 = tmp->next;

  while(tmp1)
  {
    tmp2 = tmp1;
    tmp1 = tmp1->next;

    if(tmp->val == tmp2->val) delete_node(&head, tmp2);
  }

  tmp = tmp->next;
 }
}

Logic is simple. The outer loop takes a node in every iteration starting from the first node and compares its value will rest of the right nodes in the list. If the value of any other node matches with outer loop node, the inner loop deletes that node from the linked list. The delete_node() function is used to delete the node. When the outer loop reaches end of the linked list, the linked list becomes duplicate entry free. The time complexity of the remove_duplicate() function is O(n₂).

Note: The code of this article is compiled with gcc compiler on CentOS 7 system.

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