Video Game

You are the well known video game player of your generation. You are deep diving into a newly launched video game. The game has $n$ levels each level is labelled by a number from $0$ to $n-1$. In the game some levels. In the game some of level need some prerequisites i.e two integer $x$, $y$ which means before playing the level $x$ you need to complete level $y$. For example the prereqsites {1,2