question: public static int [ ] minus ( int y1 [ ] , int y2 [ ] ) { int keep [ ] = new int [ Math . max ( y1 . length , y2 . length ) ] ; int i = 0 , j = 0 , k = 0 ; while ( j < y1 . length && k < y2 . length ) { if ( y1 [ j ] == y2 [ k ] ) { j ++ ; k ++ ; } else if ( y1 [ j ] < y2 [ k ] ) { keep [ i ++ ] = y1 [ j ] ; j ++ ; } else { k ++ ; } } while ( j < y1 . length ) { keep [ i ++ ] = y1 [ j ++ ] ; } return Arrays . copyOf ( keep , i ) ; }
target:  Minus aka Set Difference, e.g., [3,4,7,9] \ [3,7] = [4,9].