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Remove duplicates from linked list

Question:
Write a program to remove all duplicates from a singly linked list.

For example, if the list is
2-->4--->5--->7--->2---->5--->25--->2

after deletion, the output must be something like this
2-->4-->5-->7-->25


An easy solution would be to take one node at a time, compare its value with all the other nodes, and delete if there is a match. But that would be expensive.

A better solution is to sort the list and then compare adjacent values.

 Here is how we do it.
  1. Take a sorted list
  2. Compare a node with its previous node.
  3. If they have same value, delete the node
  4. Move to next node
  5. Repeat steps 2 to 4 until end of list
But you should be careful in step 4. Because if you say, node->next, you may use a dangling pointer.

For sorting the list, you can use any algorithm. Insertion sort is easiest for linked lists.

Let us look at the code.


void remove_duplicates(NODEPTR head)
{
    head = sort_list(head);
    NODEPTR temp=head;
    NODEPTR prev_node  = temp;
    temp = temp->next;
    while(temp!=NULL)
    {
        if(temp->n == prev_node->n)
        /* we have duplicate. Delete it*/
        {
             delete_nextnode(prev_node);             
             temp = prev_node->next;
        }else
        {
            prev_node = temp;
            temp = temp->next;
         }
    }
}

We are starting from second node and comparing each node with its previous node. Initial value of prev_node is head and temp is second node. When the values are equal we are calling delete_nextnode() function which will delete the next node of prev_node. Then we move to next node, not using temp = temp->next but using temp = prevnode->next.

If there is no match, we just move to next node.

delete_nextnode() used here is a simple function, which deletes the next node of its parameter. Here is the code for it.


void delete_nextnode(NODEPTR temp)
{
     if(temp->next)
     {
        NODEPTR d1 = temp->next;
        temp->next = temp->next->next;
 free(d1);
     }
}

You can download the driver program from here.

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