Problem with insertion in Binary Search Tree

Question

I have written a code for insertion in Binary Search Tree and its traversal.

class node
{
public:
    int data;
    node *left;
    node *right;
};
node* createNode(int value)
{
    node *temp = new node;
    temp->data = value;
    temp->left = NULL;
    temp->right = NULL;
    return temp;
}

node *start = NULL;

void insertNode(int val)
{
    if (start == NULL)
    {
        start = createNode(val);
        return;
    }

    node *temp = start;
    while ((temp->left != NULL) && (temp->right != NULL))
    {
        if (val < temp->data)
        {
            temp = temp->left;
        }
        else if (val > temp->data)
        {
            temp = temp->right;
        }
        else
        {
            cout << "Already exists in tree\n";
            return;
        }
    }
    if (val < temp->data)
    {
        temp->left = createNode(val);
        return;
    }
    else
    {
        temp->right = createNode(val);
        return;
    }


}

void inorder(node *root)
{
    if (root != NULL)
    {
        inorder(root->left);
        printf("%d \n", root->data);
        inorder(root->right);
    }
}

It does not work fine on some test cases.

For example, if insert 15, 25 and then 35, and then traverse the tree, it only prints 15 and 25.

I am not able to find out the problem in the code. What is the issue with my insertion logic?

Answer 1

Let's go through the behavior -

---

1. you insert the 15. if (start == NULL)
   • this check creates the start node. Now there is a start node with value 15 and left and right as NULL .

---

2. you insert 25. (temp->left != NULL) && (temp->right != NULL)
   • this turns out to be false.
   • (val < temp->data) this check creates a right node.

---

3. you insert 35. (temp->left != NULL) && (temp->right != NULL)
   • still turns out to be false.
   • (val < temp->data) this check creates a right node (replacing the current right node). Which is not right.

---

You need to correct the while loop condition here.