World four hundred and twenty-one is with almost half of the boxes already placed, but a hard puzzle.
Step by step solution
Go LEFT, UP, UP, RIGHT, RIGHT, push UP and UP. Go DOWN, DOWN, LEFT, push UP, go RIGHT, UP, push LEFT and LEFT. Go UP, UP, RIGHT, push DOWN, DOWN, go RIGHT, DOWN, DOWN, LEFT, LEFT, push UP and UP. Go RIGHT, RIGHT, DOWN and push LEFT. Go DOWN, LEFT, DOWN, DOWN, LEFT, push UP and UP. Go RIGHT, DOWN, DOWN, RIGHT, push UP and UP. Go RIGHT, UP, UP, LEFT, LEFT, push DOWN and DOWN. Go RIGHT, RIGHT, RIGHT, DOWN, RIGHT, RIGHT, UP, push LEFT, LEFT and LEFT. Go UP, UP, RIGHT, RIGHT and push DOWN. Go UP, LEFT, LEFT, DOWN, DOWN and push LEFT. Go RIGHT, UP, UP, LEFT, LEFT, DOWN, push RIGHT, go DOWN, RIGHT, push UP, go LEFT, UP and push RIGHT. Go DOWN, DOWN, LEFT, DOWN, DOWN, LEFT, LEFT, UP, UP and push RIGHT.
Go LEFT, DOWN, DOWN, RIGHT, RIGHT, UP, push UP, UP, go LEFT, UP and push RIGHT. Go DOWN, DOWN, RIGHT, RIGHT, DOWN, RIGHT, RIGHT, UP, push LEFT, LEFT, LEFT, LEFT, go DOWN, DOWN, LEFT, LEFT, UP, UP, push RIGHT, go LEFT, DOWN, DOWN, RIGHT, RIGHT, UP, push UP, go LEFT, UP and push RIGHT. Go UP, UP, UP, UP, RIGHT, RIGHT, RIGHT, DOWN, push DOWN, DOWN and DOWN. Go UP, UP, UP, UP, LEFT, LEFT, LEFT, DOWN, DOWN, DOWN, DOWN, DOWN, RIGHT, RIGHT, DOWN, RIGHT, RIGHT, UP, push LEFT, LEFT, LEFT and LEFT. Go UP, UP, UP, UP, UP, RIGHT, RIGHT, UP, UP, RIGHT, push DOWN, DOWN, DOWN, DOWN, DOWN and DOWN. Go UP, UP, UP, UP, LEFT, LEFT, LEFT, DOWN, LEFT, LEFT, UP, push RIGHT, RIGHT, RIGHT and RIGHT.
Go LEFT, LEFT, DOWN, DOWN, DOWN, LEFT and push UP. Go DOWN, RIGHT, DOWN, DOWN, DOWN, DOWN, LEFT, LEFT, UP and push RIGHT. Push UP, UP and UP. Go DOWN, DOWN, RIGHT, RIGHT, RIGHT, DOWN, RIGHT, RIGHT, UP, push LEFT, LEFT, LEFT, LEFT, go UP, LEFT, push DOWN, go LEFT, DOWN, DOWN, RIGHT, push UP, UP and UP. Go RIGHT, UP, UP, UP, UP, RIGHT, RIGHT, UP, RIGHT, push DOWN, DOWN, DOWN, DOWN and DOWN. Go UP, UP, UP, UP, LEFT, LEFT, LEFT, DOWN, DOWN, DOWN, DOWN, DOWN, RIGHT, RIGHT, DOWN, RIGHT, RIGHT, UP, push LEFT, LEFT, LEFT, LEFT, go UP, LEFT, push DOWN, go LEFT, DOWN, DOWN, RIGHT, push UP and UP.
Go RIGHT, UP, UP, UP, UP, UP, LEFT, LEFT, push LEFT, LEFT, go DOWN, LEFT, LEFT, UP, push RIGHT, RIGHT, RIGHT, RIGHT, RIGHT, RIGHT, RIGHT, RIGHT, go UP, RIGHT, push DOWN, DOWN, DOWN, DOWN and DOWN. Go UP, UP, UP, UP, LEFT, LEFT, LEFT, DOWN, DOWN, DOWN, DOWN, DOWN, LEFT, DOWN, DOWN, RIGHT, push UP, UP and UP. Go DOWN, RIGHT, RIGHT, DOWN, RIGHT, RIGHT, UP, push LEFT, LEFT, LEFT, LEFT, go DOWN, LEFT, LEFT, UP, push RIGHT, go DOWN, RIGHT and push UP. Go RIGHT, RIGHT, RIGHT, UP, UP, UP, UP, UP, LEFT, LEFT, LEFT, LEFT, LEFT, DOWN, push RIGHT, go UP, RIGHT and push DOWN.
Congratulations. You have completed world four hundred and twenty-one. Get ready for world four hundred and twenty-two.