public void stepSystem(int nstep){
  root=null;
  makeTree(nstep);
  for (Enumeration<Body> e=bodyTabRev.elementsRev(); e.hasMoreElements(); ) {
    Body b=e.nextElement();
    b.hackGravity(rsize,root);
  }
  vp(bodyTabRev,nstep);
}

public void stepSystem ( int nstep ) { root = null ; makeTree ( nstep ) ; for ( Enumeration < Body > e = bodyTabRev . elementsRev ( ) ; e . hasMoreElements ( ) ; ) { Body b = e . nextElement ( ) ; b . hackGravity ( rsize , root ) ; } vp ( bodyTabRev , nstep ) ; }
advance the n - body system one time - step .
