tree checkrots( t )
tree t;

/*** check need for rotations ***/
{ int wl, wll, wr, wrr;
  if( t != NULL ) {
    wl = wt( t->left );
    wr = wt( t->right );
    if( wr > wl ) {
        /*** left rotation needed ***/
        wrr = wt( t->right->right );
        if( wrr > wl && 2*wrr >= wr )
            { t = lrot( t );  t->left = checkrots( t->left ); }
        else if( wr-wrr > wl ) {
            t->right = rrot( t->right );
            t = lrot( t );
            t->left  = checkrots( t->left );
            t->right = checkrots( t->right );
            }
        }
    else if( wl > wr ) {
        /*** right rotation needed ***/
        wll = wt( t->left->left );
        if( wll > wr && 2*wll >= wl )
            { t = rrot( t );  t->right = checkrots( t->right ); }
        else if( wl-wll > wr ) {
            t->left  = lrot( t->left );
            t = rrot( t );
            t->left  = checkrots( t->left );
            t->right = checkrots( t->right );
            }
        }
    }
  return( t );
}