USACO Forum - Latest posts

What should I do if I can't solve anything in module

@Babez — Thu, 19 Mar 2026 14:30:37 +0000

I’m at the Bronze level and can’t solve any of the problems in the “Introduction to Sorting” module. How can I complete this module?

Issue with USACO guide's solution for lasers and mirrors

@Benq Ben Q — Tue, 17 Mar 2026 22:35:29 +0000

One way to fix this is to check whether (dir, coord) was already processed before running the for loop.

Issue with USACO guide's solution for lasers and mirrors

@Benq Ben Q — Tue, 17 Mar 2026 22:34:09 +0000

You’re right.

Issue with USACO guide's solution for lasers and mirrors

@porotun — Tue, 17 Mar 2026 22:22:33 +0000

In this problem, the usaco guide solution’s claims to be O(N), by doing a BFS and checking whether or not to update each point that shares the x value or the y value for the element at the front of the queue.

However, what if all N points were on the same line? Consider (1, 2), (1,3), (1,4)...(1, 100001) as the mirror positions, and (1,1) and (1, 100002) to be the starting and ending positions.

Although the dist would be found after processing the first point, it would do O(N) checks to update everything on the line for each element in the queue, making it O(N^2).

Am I missing something?

About Resources in Usaco Guide

@Benq Ben Q — Wed, 11 Mar 2026 19:59:03 +0000

Up to you – sometimes the resources cover essentially the same material, so it suffices to read only one, sometimes not.

About Resources in Usaco Guide

@Ashutosh_Kumar Ashutosh Kumar — Wed, 11 Mar 2026 19:47:02 +0000

Should i Read the all resources which is provide in each section