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Insert a node into sorted list

A singly linked list is a data structure where each node is linked to the next node. In such a list, if you have to add a new node, it can be done in a simple way such as


    find the last node
    link last node to new node

Which can be done using code such as

 NODEPTR insertnode(NODEPTR head, NODEPTR newnode)  
{
if(head==NULL)
{
head = newnode;
}
else
{
NODEPTR temp = head;
while(temp->next !=NULL)//till we have not reached last node
{
temp = temp->next;
}
// now temp is our last node. add new node after this
temp->next = newnode;
}

But if we have a sorted list and we want to add a new node to this list
  • we should traverse to first node temp which is greater than new node
  • add the new node to previous node of this
    • prev->next = newnode;
    • newnode->next = temp;
  • To find previous node, each time store current node in prev before moving to next node
We also need to consider one special case, where the newnode is smaller than head.
  • If the first node greater than newnode is head
  • Add the newnode before head
    •  newnode->next = head
    • head = newnode
Here is our code
 NODEPTR insert_into_sorted_list(NODEPTR head, NODEPTR newnode)  
{
NODEPTR temp = head;
NODEPTR prev = temp;
while(temp && temp->n < newnode->n)
{
prev = temp;
temp = temp->next;
}
//special case. insert before head
if(temp==head)
{
newnode ->next = head;
head = newnode;
return head;
}
//temp is bigger node. New node should be inserted before temp
//after prev. when we move to next node, we are saving node in prev
prev->next = newnode;
newnode->next = temp;
return head;//does not change head actually in this case
}

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