typedef struct avltreenode *avltree;
typedef struct avltreenode{
    int data;
    avltree left;
    avltree right;
    int height;
};

int getheight(avltree a)
{
    if(a==NULL) return -1;
    return a->height;
}

//a必須有一個左子結點b
//a與b做左單旋，更新a與b高度，返回新的根結點b
avltree singleleftrotation(avltree a)
{
    avltree b=a->left;
    a->left=b->right;
    b->right=a;
    a->height=max(getheight(a->left),getheight(a->right))+1
 
;
    b->height=max(getheight(b->left),a->height)+1;

    return b;
}

//右單旋
avltree singlerightrotation(avltree a)
{
    avltree b=a->right;
    a->right=b->left;
    b->left=a;

    a->height=max(getheight(a->left),getheight(a->right))+1;
    b->height=max(getheight(b->right),a->height)+1
 
;

    return b;
}

//左右雙旋
//a必須有一個左子結點b，且b必須有一個右子結點c
//a。b，c,做兩次單旋，返回新的根結點c
//先對b和c做右單旋。在對a和c做左單旋
avltree doubleleftrightrotation(avltree a)
{
    a->left=singlerightrotation(a->left);

    return singleleftrotation(a);
}

//右左雙旋
avltree doublerightleftrotation(avltree a)
{
    a->right=singleleftrotation(a->right);

    return
 
 singlerightrotation(a);
}

avltree avl_insertion(int x,avltree t)
{
    /*將X插入avl樹T中，並返回調整後的avl樹*/
    if(!t)/*插入空樹，則新建包括一個結點的樹*/
    {
        t=(avltree)malloc(sizeof(struct avltreenode));
        t->data=x;
        t->height=0;
        t->left=t->right=NULL;
    }/*插入空樹結束*/
    else if(x<t->data)
    {
        t->left=avl_insertion(x,t->left);
        if(getheight(t->left)-getheight(t->right)==2)
        {
            /*須要左旋*/
            if(x<t->left->data)
                t=singleleftrotation(t);//左單旋 singleleftrotation
            else
                t=doubleleftrightrotation(t);//左右雙旋
        }
    }
    else if(x>t->data)
    {
        t->right=avl_insertion(x,t->right);
        if(getheight(t->left)-getheight(t->right)==-2)
        {
            /*須要右旋*/
            if(x>t->right->data)
                t=singlerightrotation(t);//右單旋
            else
                t=doublerightleftrotation(t);//右左雙旋
        }
    }

    //x=data，無須插入
    t->height=max(getheight(t->left),getheight(t->right))+1;
    return t;
}

平衡二叉樹的調整模版