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Delete a node from doubly linked list

Deletion operation in DLL is simpler when compared to SLL. Because we don't have to go in search of previous node of to-be-deleted node. 

Here is how you delete a node
  1. Link previous node of node of to-be-deleted to next node.
  2. Link next node of node of to-be-deleted to previous node.
  3. Free the memory of node of to-be-deleted
Simple, isn't it. The code can go like this.
  1. prevnode = delnode->prev;
  2. nextnode = delnode->next;
  3. prevnode->next = nextnode;
  4. nextnode->prev = prevnode;
  5. free(delnode);
And that is it. The node delnode is deleted.

But we should always consider boundary conditions. What happens if we are trying to delete the first node or last node?

If first node is to be deleted, its previous node is NULL. Hence step 3 should not be used.  And also, once head is deleted, nextnode becomes head.

Similarly if last node is to be deleted, nextnode is NULL. Hence step 4 is as strict NO NO. And we should set prevnode to tail.

After we put these things together, we have
  1. prevnode = delnode->prev;
  2. nextnode = delnode->next;
  3. if(prevnode) prevnode->next = nextnode;
  4. if(nextnode) nextnode->prev = prevnode;
  5. if(delnode==head) head = nextnode;
  6. if (delnode==tail) tail = prevnode;
  7. free(delnode); 
 Are we ready for the code now? Here it is


  1.  
 void delete_node(NODEPTR*head, NODEPTR *tail,int value)  
 {  
    NODEPTR prevnode,nextnode;  
    NODEPTR delnode = find_node(*head,value);  
    if(delnode==NULL)  
    {  
       printf("NO such node..");  
       return;  
    }  
    prevnode = delnode->prev;  
    nextnode = delnode->next;  
    if(prevnode) //we have a previous node  
      prevnode->next = nextnode;  
    if(nextnode) //we have a next node  
      nextnode->prev = prevnode;  
    //if head is deleted, adjust head  
    if(delnode==*head)   
      *head = nextnode;  
    //if tail is deleted, adjust tail  
    if(delnode==*tail)   
      *tail = prevnode;  
    free(delnode);  
 }

You can download the complete program here.
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