通过 c 中的 function 中的指针传递参数值

Question

I am a newbie in learning c..... So I am trying to create a tree structure from inputs like:

(2, 50) (4, 30) (9, 30) (10, 400) (-5, -40)
(7, 20) (19, 200) (20, 50) (-18, -200) (-2, 29)
(2, 67) (4, 35) (9, 45) (-18, 100) 

Firstly, I have defined some data structures and auxiliary function as below:

typedef struct AVLTreeNode
{
    int key;                    //key of this item
    int value;                  //value (int) of this item
    int height;                 //height of the subtree rooted at this node
    struct AVLTreeNode *parent; //pointer to parent
    struct AVLTreeNode *left;   //pointer to left child
    struct AVLTreeNode *right;  //pointer to right child
} AVLTreeNode;

//data type for AVL trees
typedef struct AVLTree
{
    int size;          // count of items in avl tree
    AVLTreeNode *root; // root
} AVLTree;

// create a new AVLTreeNode
AVLTreeNode *newAVLTreeNode(int k, int v)
{
    AVLTreeNode *newNode;
    newNode = malloc(sizeof(AVLTreeNode));
    assert(newNode != NULL);
    newNode->key = k;
    newNode->value = v;
    newNode->height = 0;  // height of this new node is set to 0
    newNode->left = NULL; // this node has no child
    newNode->right = NULL;
    newNode->parent = NULL; // no parent
    return newNode;
}

// create a new empty avl tree
AVLTree *newAVLTree()
{
    AVLTree *T;
    T = malloc(sizeof(AVLTree));
    assert(T != NULL);
    T->size = 0;
    T->root = NULL;
    return T;
}

Then I wrote a function to insert each pair of data into the tree:

int InsertNode(AVLTree *T, int k, int v)
{
    AVLTreeNode *currentNode = T->root;
    AVLTreeNode *parent;
    //if the tree/subtree is empty
    if (currentNode == NULL)
    {
        currentNode = newAVLTreeNode(k, v);
    }
    //keys are equal
    else
    {
        while (currentNode != NULL)
        {
            parent = currentNode;
            if (k == currentNode->key)
            {
                if (v == currentNode->value)
                {
                    return 0;
                }
                else if (v < currentNode->value)
                {
                    currentNode = currentNode->left;
                }
                else if (v > currentNode->value)
                {
                    currentNode = currentNode->right;
                }
            }
            else if (k < currentNode->key)
            {
                currentNode = currentNode->left;
            }
            else if (k > currentNode->key)
            {
                currentNode = currentNode->right;
            }
        }

        currentNode = newAVLTreeNode(k, v);
        currentNode->parent = parent;
    }
    T->size++;
    return 1;
}

I was expecting the values k and v in the function arguments would pass onto T->root->key, and T->root->value respectively, but it doesn't work..... Could someone help me to correct the code and explain to me the reason why my code is wrong please?

Answer 1

The main problem is that your InsertNode() function simply doesn't. It does construct an AVLTreeNode , traverse the tree to the new node's insertion point, and set its parent pointer, but it never updates a child pointer of the parent node, nor the root pointer of the tree in the case that it is initially empty. In particular, assigning to currentNode does not have this effect.

However, you could make this code work by adjusting it for currentNode to be a double pointer ( AVLTreeNode ** ), so that you can assign indirectly through it to the wanted root or child pointer.

AVLTreeNode **currentNode = &T->root;

// ...

currentNode = &(*currentNode)->left;

// ...

// So that you can ultimately do this:
*currentNode = newAVLTreeNode(k, v);