if length is even . so it is our first case we have two choices
if ( len % 2 == 0 ) { q . add ( curr . substring ( 0 , len / 2 ) + "0" + curr . substring ( len / 2 ) ) ; q . add ( curr . substring ( 0 , len / 2 ) + "1" + curr . substring ( len / 2 ) ) ; }