203A - Two Problems

A simple simulation problem, but be careful that one could solve 0, 1, or 2 problems.

203B - Game on Paper

Once a square becomes black, we check all the 3 * 3 areas (in fact there 9 such areas) which contain it, to see whether they are all black. The complexity is O(M).

203C - Photographer

A straightforward greedy algorithm solves this problem.

203D - Hit Ball

We can consider each of the three dimensions independently. At first, we can calculate when it hits the door according to v_y. Then, we calculate the total distance that it goes along the x and y dimension, respectively, assuming that no reflection occurs. It can be observed that for x dimension, the distance has a cycle of length 2a while for y dimension, the cycle has length 2b. Therefore, we “eliminate” a multiple of cycles from the total distance and it is then simple to determine the final position, according to the “residual” distance.