AVL Tree insertion (in C) failed

Question

I learnt AVL tree from Data Structures and Algorithms Analysis in C , I typed code myself, and its insertion function cannot work well.

I checked these functions with many data. Some nodes cannot be inserted and some nodes are inserted randomly. Unsorted, I mean.

Here is part of my code:

AVLTree.h:

/* Data structures model */
typedef int data_type;
typedef struct avlnode {
    data_type data;
    int height;
    struct avlnode *lchild;
    struct avlnode *rchild;
} AvlNode;
typedef AvlNode AvlTree;

/* Function Prototypes including init, find(custom/min/max) and insert */
AvlTree *AVL_create(data_type value);
AvlNode *AVL_find(AvlTree *tree, data_type value);
AvlNode *AVL_find_min(AvlTree *tree);
AvlTree *AVL_find_max(AvlTree *tree);
AvlTree *AVL_insert(AvlTree *tree, data_type value);
#define MAX_HEIGHT(x,y) (x > y) ? x : y

AVLTree.c

/* Static function to get the height of a node in the tree */
static int height(AvlNode *node) {
    return (node == NULL) ? -1 : node->height;
}

/* Tree init func with a valued root node */
AvlTree *AVL_create(data_type value) {
    AvlTree *newtree;
    newtree = (AvlTree *)malloc(sizeof(AvlTree));
    if (newtree == NULL)
        return NULL;

    newtree->lchild = NULL;
    newtree->rchild = NULL;
    newtree->height = 0;
    newtree->data = value;
    return newtree;
}

/* Node search functions. In fact I use BST search functions here */ 
/* I'm not sure could them run well here in AVL tree */

AvlNode *AVL_find(AvlTree *tree, data_type value) {
    AvlTree *temptree = tree;
    if (temptree == NULL)
        return NULL;
    if (value < temptree->data)
        return AVL_find(tree->lchild, value);
    else if (value > temptree->data)
        return AVL_find(tree->rchild, value);
    else
        return temptree;
}
AvlNode *AVL_find_min(AvlTree *tree) {
    AvlTree *temptree = tree;
    if (temptree != NULL) {
        while (temptree->lchild != NULL)
            temptree = temptree->lchild;
    }
    return temptree;
}
AvlTree *AVL_find_max(AvlTree *tree) {
    AvlTree *temptree = tree;
    if (temptree != NULL) {
        while (temptree->rchild != NULL)
            temptree = temptree->rchild;
    }
    return temptree;
}


AvlTree *AVL_insert(AvlTree *tree, data_type value) {
    AvlTree *temptree = tree;

    if (temptree == NULL) {
        temptree = (AvlNode *)malloc(sizeof(AvlNode));
        if (temptree == NULL)
            return NULL;
        else {
            temptree->data = value;
            temptree->height = 0;
            temptree->lchild = NULL;
            temptree->rchild = NULL;
        }
    }
    else if (value < temptree->data) {
        temptree->lchild = AVL_insert(temptree->lchild, value);
        if (height(temptree->lchild) - height(temptree->rchild) == 2) {
            if (value < temptree->lchild->data)
                temptree = single_rotate_with_left(temptree);
            else
                temptree = double_rotate_with_right_left(temptree);
        }
    }
    else if (value > temptree->data) {
        temptree->rchild = AVL_insert(temptree->rchild, value);
        if (height(temptree->rchild) - height(temptree->lchild) == 2) {
            if (value > temptree->rchild->data)
                temptree = single_rotate_with_right(temptree);
            else
                temptree = double_rotate_with_left_right(temptree);
        }
    }
    temptree->height = MAX_HEIGHT(height(temptree->lchild), height(temptree->rchild)) + 1;
    return temptree;
}

main.c

#include "AVLTree.h"

int main() {
    AvlTree *newtree = AVL_create(50);

    AVL_insert(newtree, 70);
    AVL_insert(newtree, 80);
    AVL_insert(newtree, 90);

    for (int i = -5; i < 20; i++) {
        AVL_insert(newtree, i * i * i);
    }

    printf("root node: %d\n", newtree->data);
    printf("left of root node: %d\n", newtree->lchild->data);
    printf("findmin: %d\n", AVL_find_min(newtree)->data);
    printf("findmax: %d\n", AVL_find_max(newtree)->data);
    return 0;
}

Answer 1

I have tried your program and disabled the rebalancing (see below where). Then it works correctly, printing out:

root node: 50
left of root node: -125
findmin: -125
findmax: 6859

Which is correct I think.

So I think the problem is in one of your rotation functions.

If you can't find the problem, please show them to us.

else if (value < temptree->data) {
    temptree->lchild = AVL_insert(temptree->lchild, value);
    /*if (height(temptree->lchild) - height(temptree->rchild) == 2) {
        if (value < temptree->lchild->data)
            temptree = single_rotate_with_left(temptree);
        else
            temptree = double_rotate_with_right_left(temptree);
    }*/
}
else if (value > temptree->data) {
    temptree->rchild = AVL_insert(temptree->rchild, value);
    /*if (height(temptree->rchild) - height(temptree->lchild) == 2) {
        if (value > temptree->rchild->data)
            temptree = single_rotate_with_right(temptree);
        else
            temptree = double_rotate_with_left_right(temptree);
    }*/
}