AVL 树插入更改根

Question

I have to implement an AVL Tree in C. I wrote the following insert-Method, but it only keeps changing the root of the tree.

I think it has something to do with how I go through the tree. I know how to do it when only a node is needed for the function, but how do I do it with an entire tree?

void AVL_insert(AVLTree* avlt, int value){

    AVLTree *tree = avlt;
    AVLNode *root = avlt->root;

    if(tree->root == NULL){
        AVLNode *node = calloc(1, sizeof(AVLNode));
        node->value = value;
        node->height = 0;
        tree->root = node;
        tree->numberOfNodes = avlt->numberOfNodes+1;
        return;
    }
    else if(value < tree->root->value){
        tree->root = root->left;
        AVL_insert_value(tree, value);
    } 
    else if(value > tree->root->value){
        tree->root = root->right;
        AVL_insert_value(tree, value);
    } 
    else return;
    

//..add height, balance tree

}

Structs:

struct AVLNode{
struct AVLNode* left;       
struct AVLNode* right;  
struct AVLNode* parent;
int value;
int height;
};

struct AVLTree{
struct AVLNode* root;
int numberOfNodes;
};

typedef struct AVLNode AVLNode;
typedef struct AVLTree AVLTree;

Answer 1

My guess what is wrong.

1. Inserting to an empty node allocates the root, it's left and right children are set to NULL.

AVLNode *node = calloc(1, sizeof(AVLNode));

calloc() set the content of the node to zeros.

2. A new value is inserted. As a child is updated before recursion, the root is set to its left or right child. Both of them are NULL.

tree->root = root->left; // root->left is `NULL`

Now, the root is NULL.

3. Adding a next value. The root is NULL. Go to point 1.

As result a new value is alternately added to either empty or 1-element tree.

Possible fix: Do not update root's children. Just insert a value into the tree as leaf and let re-balancing code optimize tree's depth while returning from recursion.