Error when working with pointers in insertion of a node in a binary search tree

Question

So I have the following code:

#include <iostream>
#include <vector>
#include <cstdlib>
#include <ctime>
using namespace std;
struct Node
{
    int value;
    Node *left = NULL;
    Node *right = NULL;
    Node(int value)
    {
        this->value = value;
    }
};
struct BST
{
    Node *root = NULL;
    void insert(int value)
    {
        cout<<"Inserting: "<<value<<endl;
        Node *current = root;
        while(current != NULL)
        {
            cout<<"YEA";
            if(value > current->value)
            {
                current = current->right;
            }
            else current = current->left;
        }
        current = new Node(value);
        cout<<"In the node val: "<<current->value<<endl;
        if(root == NULL)
        {
            cout<<"Root is NULL but it shouldn't\n";
        }
        cout<<"Root val: "<<root->value<<endl;

    }
    void remove(int value)
    {
        Node *toReplace = NULL;
        Node *toBeReplacedWith = NULL;
        toReplace = search(value);

        Node *current = toReplace->left;
        if(current == NULL) toBeReplacedWith = toReplace->right;
        else
        {
            while(current->right != NULL)
            {
                current = current->right;
            }
            toBeReplacedWith = current;
        }
        current->value = toBeReplacedWith->value;
        current->left = toBeReplacedWith->left;
        current->right = toBeReplacedWith->right;
        free(toBeReplacedWith);
    }
    Node* search(int value)
    {
        Node *current = root;
        while(current != NULL && current->value != value)
        {
            if(current->value > value) current = current->left;
            else current = current->right;
        }
        if(current == NULL)
        {
            cout<<"The node didn't exist in the BST";
        }
        return current;
    }
    void traverse()
    {
        rec_traverse(root);
    }
private:
    void rec_traverse(Node * current)
    {
        if(current == NULL) return;
        rec_traverse(current->left);
        cout<<current->value<<endl;
        rec_traverse(current->right);
    }
};
int main()
{
    BST tree;
    for(int i = 0; i < 10; ++i)
    {
        tree.insert(i);
    }
    tree.traverse();
    return 0;
}

Can somebody tell me why when inserting an element the root still points to NULL instead to a Node instance? I even check the current pointer for a value and it has but still for some reason root is NULL when it should be the first node to be assigned a value

Answer 1

Can somebody tell me why when inserting an element the root still points to NULL

Your function BST::insert(int value) at no point modifies root .

That's why root remains NULL.

One way to make your approach work is to have current point to the pointer you want to modify, instead of having current hold a copy of that pointer.

Answer 2

You are changing the Node* that current is pointing to, but you never touch the root.

current = new Node(value); should be root = new Node(value); if root was null.

Furthermore, as an aside, apart from the question here, your code will be a LOT simpler if you use recursive calls for insertion and deletion (and you wouldn't have to worry about the case when root is pointing to null because it will be implicitly handled)