Recursive insertion of BST

Question

I have made a function for insertion in BST using loops and it is working perfectly fine. Now, when iam writing to do it using recursion i don't know why it's not working properly, however the logic is correct according to me. It seems that no newnode is being added to the BST tree and head of the tree after coming out of the insertion function is again becoming NULL.

#include <iostream>
using namespace std;

class node{
public:
   int data;
   node *right;
   node *left;
   node(){
      data=0;
      right=NULL;
      left=NULL;
   }
};

class tree{
   node *head;
   int maxheight;
   void delete_tree(node *root);
public:
   tree(){head=0;maxheight=-1;}
   void pre_display(node* root);
   node* get_head(){return head;}
   void insert(int key,node* current);
};

void tree::insert(int key,node *current){
   if(current==NULL)
   {
      node *newnode=new node;
      newnode->data=key;
      current=newnode;
   }
   else{
      if(key<current->data)
         insert(key,current->left);
      else
         insert(key,current->right);
   }
   return;
}

void tree::pre_display(node *root){
   if(root!=NULL)
   {
      cout<<root->data<<" ";
      pre_display(root->left);
      pre_display(root->right);
   }
}

int main(){
   tree BST;
   int arr[9]={17,9,23,5,11,21,27,20,22},i=0;

   for(i=0;i<9;i++)
   BST.insert(arr[i],BST.get_head());

   BST.pre_display(BST.get_head());
   cout<<endl;

   system("pause");
   return 0;
}

Please tell me what should i change in the algorithm to make it work.

Answer 1

In your insert function

void tree::insert(int key,node *current){
    if(current==NULL)
    {
        node *newnode=new node;
        newnode->data=key;
        current=newnode;
    }
    else{
        if(key<current->data)
            insert(key,current->left);
        else
            insert(key,current->right);
    }
    return;
}

you allocate a new node but never set BST::head to newly allocated head. So BST::get_head will always return null.

One way to fix this would be for insert to return a node. This would be root node in your case and set the BST::head to this value.

Answer 2

Your recursion looks fine, but you don't actually add the node anywhere! You just recurse through the tree.

Edit You can change the insert method to take a pointer to a pointer, like this:

void tree::insert(int key, node **current)
{
    if(*current == NULL)
    {
        node *newnode = new node;
        newnode->data = key;
        *current = newnode;
    }
    else 
    {
        if(key < (*current)->data)
            insert(key, &(*current)->left);
        else
            insert(key, &(*current)->right);
    }
}

And in main call it like this:

BST.insert(arr[i], &BST.get_head());  // Note the ampersand (&)