protected void SWAP(int a,int b,Stack<d_node> cover_set){
  d_node tmp=cover_set.element(a);
  cover_set.set(a,cover_set.element(b));
  cover_set.set(b,tmp);
}

protected void SWAP ( int a , int b , Stack < d_node > cover_set ) { d_node tmp = cover_set . element ( a ) ; cover_set . set ( a , cover_set . element ( b ) ) ; cover_set . set ( b , tmp ) ; }
swap two nodes in a cover set .
