Problem A :

The ball is invisible for time: [S, T], which means it will be invisible for the section: [S*V, T*V]. To hit the ball by Akoi D must not lie in this section, that means D should be less than S * V or greater than T * V

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Problem B:

Just a straight forward implementation, print those numbers which are not equal to X.

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Problem C:

Key Point: There are no white cells inside the polygon of black cells, so, you can assume the polygon as a contiguous block of black cells, whose sides we have to find.

To find sides: For every black cell X, check its adjacent (left, right, up & down) cels (let's say each cell Y), we have to check the followings:

1. If Y is black then it doesn't contribute to the answer.

2. If Y is white, then check whether that edge is already added or not. If it is not added then add this side to the answer.

To check some side is added or not, you can use loops or maintain some data structure like set (in C++). I've used the set in my code.

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Problem D:

Consider a vertical line that passes through the center of the circle (h, k). Iterate on the integer points (let say each point P) of that line such that those points lie inside the circle i.e. points: (h, k-r) to (h, k+r), r is the radius of the circle. For each point P, find the number of integer points on the left and right of P such that they lie inside the circle.

To find the number of points you can use the binary search.

NOTE: Maintain the precisions.

Link to my code