How does this code work?

Question

Sample input:
5
5
1 0 15
1 0 20
1 2 78
2 2 0
3 0
Sample output:
78
2
Explanation:
There are 5 shelves and 5 requests, or queries.
- 1 Place a 15 page book at the end of shelf 0 .
- 2 Place a 20 page book at the end of shelf 0 .
- 3 Place a 78 page book at the end of shelf 2 .
- 4 The number of pages in the 0th book on the 2nd shelf is 78.
- 5 The number of books on the 0th shelf is 2.
https://code.sololearn.com/cX8498IfQAty/?ref=app