Serialize and Deserialize a Binary Tree in C

typedef struct node_ {
    int key;
    struct node_ *left, *right, *parent;
} node;

// A utility function to create a new node
node *newNode(int item) {
    node *temp =  (node *)malloc(sizeof(node));
    temp->key = item;
    temp->left = temp->right = temp->parent = NULL;
    return temp;
}

/* pre-order traversal. -1 indicates NULL node. Assumption is that a node with -1 key doesn't exist 
 * Serialized tree is saved in array, passed as argument */
void serialize(node *root, int *arr, int *sz) {
    static int idx;

    if(root==NULL)  { arr[idx++] = -1; return; }

    arr[idx++] = root->key;

    serialize(root->left, arr, sz);
    serialize(root->right, arr, sz);

    *sz = idx;  // caller will know the last index of array
}

/* Input is pre-order printed array. Return a Binary Tree 
 * NOTE: this implementation does not require to know the size of the array. It assumes are array is well formed through serialization.
 */
node *deserialize(int *arr) { 
    static int idx;
    if(arr[idx] == -1) { idx++; return NULL; }

    node *n = newNode(arr[idx++]);
    n->left = deserialize(arr);
    n->right = deserialize(arr);
    return n;
}