Question

I have written a recursive insert algorithm for a BST. However, there is a bug in the algorithm. If anyone could please give me a pointer, it will be greatly appreciated. Please not that y = NULL in the initial call.

    void insert_recursive(Node **root, Node *z, Node *y) {
    // z is the pointer to the node being inserted, and y keeps track of z's parent
    Node *x = *root;

    if (x != NULL) {
        y = x;
        if (z->val < x->val)
            insert_recursive(&(x->left), z, y);
        else
            insert_recursive(&(x->right), z, y);
    }
    else {
        if (y == NULL)
            { *r = z; printf("inserting root, %d\n", z->val); }
        else if (z->val < x->val)
             { y->left = z; printf("inserting left of %d, item %d\n", y->val, z->val); }
        else
            { y->right = z; printf("inserting right of %d, item %d\n", y->val, z->val); }
    }
} 

Answer 1

It may not be the only problem, but your line

else if (z->val < x->val)

occurs in the else clause of if (x != NULL). In other words, x is guaranteed to be NULL here.