Binary Search Tree - Insert

Question

I am trying to write a function that inserts a value into a binary tree. I got this so far:

struct node
{
     int data;
     struct node* left;
     struct node* right;
};


struct node* newNode(int data)
{
     struct node* node = (struct node*)
     malloc(sizeof(struct node));
     node->data = data;
     node->left = NULL;
     node->right = NULL;

     return(node);
}

// Insert
struct node* insert (struct node* node, int data)
{
    if (node == NULL)
        node->data=data;
    else 
    {
      if (data < node->data)
         node->left = insert(node->left, data);
      else
         node->right = insert(node->right ,data);
    }
    node->left = NULL;
    node->right = NULL;
    return node;
}

But everytime I ran it, it stops working. Therefore I think there must be something wrong with my insert function. Any ideas?

Answer 1

There are two problems as far as I can tell:

struct node* insert (struct node* node, int data)
{
    if (node == NULL)
        node->data=data;  // 1: Dereferencing a null pointer is a bad idea.
    else 
    {
      if (data < node->data)
         node->left = insert(node->left, data);
      else
         node->right = insert(node->right ,data);
    }

    // 2: Throwing away the subtrees is also a bad idea.
    node->left = NULL;
    node->right = NULL;
    return node;
}

You should use newNode to create a node in the tree, and keep the subtrees:

struct node* insert (struct node* node, int data)
{
    if (node == NULL)
        node = newNode(data);
    else 
    {
      if (data < node->data)
         node->left = insert(node->left, data);
      else
         node->right = insert(node->right ,data);
    }
    return node;
}