Book :
Difficulty Level : Medium
Submissions : 312
Asked In :
Marks :30
: 2 | : 0
You are given a book with $$$n$$$ chapters.
Each chapter has a specified list of other chapters that need to be understood in order to understand this chapter. To understand a chapter, you must read it after you understand every chapter on its required list.
Currently, you don't understand any of the chapters. You are going to read the book from the beginning till the end repeatedly until you understand the whole book. Note that if you read a chapter at a moment when you don't understand some of the required chapters, you don't understand this chapter.
Determine how many times you will read the book to understand every chapter, or determine that you will never understand every chapter no matter how many times you read the book.
The first line of each test case contains a single integer $$$n$$$ $$$(1 \le n \le 2 \cdot 10^5)$$$ — number of chapters.
Then $$$n$$$ lines follow. The $$$i_{th}$$$ line begins with an integer $$$k_{i}$$$ $$$(0 \le k_{i} \le n−1)$$$ — number of chapters required to understand the $$$i_{th}$$$ chapter. Then $$$k_{i}$$$ integers $$$a_{i,1},a_{i,2},…,a_{i,ki}$$$ $$$(1 \le a_{i,j} \le n, a_{i,j} ≠ i, a_{i,j} ≠ a_{i,l} for j≠l)$$$ follow — the chapters required to understand the i-th chapter.
It is guaranteed that the sum of $$$k_{i}$$$ do not exceed $$$2 \cdot 10^5$$$.
For each test case, if the entire book can be understood, print how many times you will read it, otherwise print −1.
5 1 2 1 3 1 4 1 5 0
5
5 1 5 1 1 1 2 1 3 1 4
-1
