AVL / Binary Search Tree

Question

I have a C programming question. The below is a insert to a node with a key.

I don't understand why node->left = insert(node->left,key)

I assume this code WILL update the node->left with what?

Isn't it calling insert() again? It's like calling the same function again and again isn't it a infinite loop or insert calling?

I checked a few example, they are all updating the node->left that way by calling the same function again? Let's say I misunderstood, what are the storing in it? The pointer? Or are they just magically linked?

// An AVL tree node
struct node
{
    int key;
    struct node *left;
    struct node *right;
    int height;
};

struct node* insert(struct node* node, int key)
{
    /* 1.  Perform the normal BST rotation */
    if (node == NULL)
        return(newNode(key));

    if (key < node->key)
        node->left  = insert(node->left, key);//This just called Insert function again?
    else
        node->right = insert(node->right, key);

Answer 1

Yes, they are calling the insert function again, but note that the arguments have changed ¹ . This is what is called recursion .

Also note that it does not always call the same function again: in case node==NULL it does not call it. So this is not infinite; as you take node->left , node->left->left and so one, you once come to a point where the pointer is NULL .

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¹ I don't say that recursive call must always change its arguments, but that's the most common approach.