Help on Blocked Billboard

Debugging Help
#1

Problem Info

Blocked Billboard: https://usaco.org/index.php?page=viewproblem2&cpid=759

Question

Why do all test cases pass at a very short time except 9 and 10? (When submitting on usaco.org, test cases 9 and 10 run at ~3500 ms.)

What I’ve Tried

I switched from using a list to a set, which made it run just under the time limit on usaco.org on cases 9 and 10, but I still want to know why my program still runs for such a long time with these cases.

My Work

squares = set()

fin = open("billboard.in", "r")
fout = open("billboard.out", "w")

for i in range(3):
    x1, y1, x2, y2 = map(int, fin.readline().split())
    
    if i == 0 or i == 1:
        for j in range(x1, x2):
            for k in range(y1, y2):
                squares.add((j, k))
    else:
        for j in range(x1, x2):
            for k in range(y1, y2):
                if (j,k) in squares:
                    squares.remove((j, k))

fout.write(str(len(squares)))

I create an empty set of visible squares. For the first two input lines (which are the billboards), I add the coordinates inside/on the billboards to my squares set as they are visible. For the last input line (which is the truck), I remove all of the coordinates inside/on the truck from the squares list if they exist, as they are no longer visible. Finally, I write the number of visible squares to billboard.out.

#2

Python is slow

You can download the test cases yourself to check what the last two tests are.