Question

I was trying to implement a simple Binary Search Tree for practice. I tried to just add values and print the values in the nodes. However, I am not getting the proper ascending order of values in the nodes. Here is what I have:

struct Node
{
    int data;
    Node* leftN;
    Node* rightN;

};

typedef Node* Node_ptr;
Node_ptr head;

//INSERT_VALUE FUNCTION
Node* insert_value(Node_ptr leaf, int key)
{
    //Root case when there is no set value yet  
    if(leaf == NULL)
    {
        leaf = new Node;
        head = leaf;
        cout << "Make the first node" << endl;
        leaf->data = key;
        leaf->leftN = NULL;
        leaf->rightN = NULL;
        return leaf;
    }   
    //Left child Node
    if(key < leaf->data)
    {
        //Search for a spot in the tree to add a Node (left value < root value < right value)
        //This is only true for the left child Node
        if(leaf->leftN != NULL)
            insert_value(leaf, key);
        //We have found a spot in the tree to add a new Node and add the value of key
        else 
        {
            cout << "Insert-left" << endl;
            leaf->leftN = new Node;
            leaf = leaf->leftN;
            leaf->data = key;
            leaf->leftN = NULL;
            leaf->rightN = NULL;
            return leaf;
        }
    }

    //Right child Node
    else if(key >= leaf->data)
    {
        //Search for a spot to add a new Node in the tree (only amongst the right child Nodes)
        if(leaf->rightN != NULL)
            insert_value(leaf, key);    
        //Once we have found a spot to add a new Node, append the new Node
        else
        {
            cout << "Insert-right" << endl;
            leaf->rightN = new Node;
            leaf = leaf->rightN;    
            leaf->data = key;
            leaf->leftN = NULL;
            leaf->rightN = NULL;
            return leaf;
        }
    }   
}

//PRINT FUNCTION
void printTree(Node_ptr leaf)
{
    if(leaf == NULL)
        return;
    printTree(leaf->leftN);
    cout << "Data element: " << leaf->data << endl;
    printTree(leaf->rightN);
}

//MAIN
int main()
{
    Node_ptr root = NULL;
    int i;

    //initialize values
    for(i = 1; i < 12; i+=2)
        root = insert_value(root, i);
    root = head;
    for(i = 0; i < 11; i+=2)
        root = insert_value(root, i);
    root = head;
    printTree(root);

    root = head;
    cout << "Head Node: " << root->data << endl;

    return 0;
}

When I printed the results, this is what I got:
0, 2, 4, 6, 8, 10, 1, 3, 5, 7, 9, 11 and the value of the head node is 1

Answer 1

Because you are calling the insertion as:

    root = insert_value(root, i);

the location at which you insert is always using a subtree starting at the last insertion. Except the time that you re-start to add the odd numbers, when you start inserting at the head.

If you create a class BinarySearchTree that contains a head pointer, and an insert method taking an int value that calls Node::insert( head, value ), then you can just call insert on that class, without passing it a node, and it can always see to it that the insertions use the root of the tree for the start of the recursion.

Just me, but I would have a constructor for Node that takes an int and initializes the pointers to NULL. That way you don’t have to do that in the insert method.