TREE* mergeTrees(TREE* root1 ,TREE* root2) // O(h), h = height of one of the trees
{
    NODE2* newRoot, fatherMinimunRight;
    TREE* newTree;

    // find the minimun of the right tree
    newRoot = findMinimum(root2);

    // the father of the minimumRight should point to null.
    fatherNewRoot = father(root2->head.left, newRoot->key);
    fatherNewRoot->left = NULL;

    // newRoot should point from right to root2
    newRoot->right = root2->head.left;

    // newRoot should point from left to root1
    newRoot->left = root1->head.left;

    // create the new tree
    if (!(newTree = (TREE*)malloc(sizeof(TREE))))  return NULL;  //  allocation error

    if (T_init(newTree) == FALSE)
    {
        return NULL;
    }

    newTree->head.left = newRoot;

    return newTree;
}