C++ Binary Search Tree Insert Implementation

Question

I'm trying to build a function to insert into a binary search tree, but I'm having a hard time figuring out why it won't work. I understand fundamentally how the function is supposed to work, but based on the template I was given it seems as though I am to avoid creating a BST class but instead rely on the Node class and build the desired functions to work on that. Here's the given template:

#include <iostream>
#include <cstddef>

using std::cout;
using std::endl;

class Node {
    int value;
public:
    Node* left;       // left child
    Node* right;      // right child
    Node* p;          // parent
    Node(int data) {
        value = data;
        left = NULL;
        right = NULL;
        p  = NULL;
    }
    ~Node() {
    }
    int d() {
        return value;
    }
    void print() {
        std::cout << value << std::endl;
    }
};

function insert(Node *insert_node, Node *tree_root){
    //Your code here
}

The issue I'm having is when I implement the following code, where getValue is a simple getter method for Node:

int main(int argc, const char * argv[]) {
     Node* root = NULL;
     Node* a = new Node(2);
     insert(a, root);
}

void insert(Node *insert_node, Node *tree_root){
    if (tree_root == NULL)      
        tree_root = new Node(insert_node->getValue());

The code appears to compile and run without error, but if I run another check on root after this, it returns NULL. Any idea what I'm missing here? Why is it not replacing root with a new node equal to that of insert_node?

I also realize this doesn't appear to be the optimal way to implement a BST, but I am trying to work with the template given to me. Any advice would be appreciated.

Answer 1

As Joachim said your issue relates to difference between passing parameter by reference and by value.

In your code void insert(Node *insert_node, Node *tree_root) you pass Node* tree_root by value. Inside the function you change local copy of this pointer, so outer value is not changed.

To fix it you should pass Node* tree_root by reference . Parameter declaration can be Node*& tree_root (or Node** tree_root ). Eg:

void insert(Node* insert_node, Node*& tree_root){
    if (tree_root == NULL)      
        tree_root = new Node(insert_node->getValue());