### D. Power Plant

Author: Suhendry Effendy

(link to problem D)

To solve this problem, let us construct a graph G’ which is equivalent to the original graph G, but also contains an additional node x and edges (x,y) with zero weight for all nodes y in G which are power plants. Find the MST of G’ and this is the answer to the problem.

Figure 1

Figure 2

Figure 1 shows an example of graph G’ while the area enclosed by the red box is the original graph G. Figure 2 shows the MST of G’ and the cost of this MST is equal to the answer to this problem.

To understand why this solution works, first, notice that all nodes in T’ (the MST of G’) are connected to node x, and node x only connected to power plants in G’. If we remove x from T’, then all the remaining nodes are still connected to the power plants. Therefore, T’ is a valid (feasible) solution to the problem. Moreover, T’ has the minimum cost since the edges connecting x to the power plants have zero weight.

are you sure there isn’t mod operation ?

and this : j = j + 1, why it isn’t j = j-1 ?

Ah, you’re right, it should be j =j – 1 (updated).

MOD operation is not needed, What we really need is whether a two substrings have a same hash value or not, not the hash value itself. There are only addition and multiplication operators, so overflowing the result doesn’t matter.

^that comment above is for pasti pas

Halo Mr. Suhendry.

Saya ingin sekali bisa berkompetisi di ACM ICPC. Saya Mahasiswa tingkat 2 di salah satu perguruan tinggi di bandung. Tapi skill koding saya tidak terlalu bagus. algoritma sorting saja masih bingung. Gimana ya kiat nya untuk belajar algoritma yg baik dan benar agar bisa nanti suatu saat ke ACM ICPC?

How to determine which prime number should be used? I tried some prime numbers and the result is not as expected.

(Problem F). Just use large prime number (larger is better, its chance to collide is smaller). I used 1000003 and it’s fine.

I have implemented it and submitted on Live Archive, but got WA 😀

Is there something wrong with my implementation? Could you please check it? http://pastebin.com/ECJX9yEa

I didn’t read your code, but it failed the first sample input: PASTIPAS.

Yes and it works if I do MOD operation, but still WA on Live Archive. Does your solution with above algorithm still work on LA? Maybe you/others have added more test cases to break that hash function? Or my implementation wrong?

I’m the one who send the data to LA and I’m pretty sure LA’s admin was quite busy to do any changes on the dataset.

BTW, I’ve just noticed an error in F’s pseudocode. There is one line which is wrong. Try to figure that out! 🙂

hint: pay attention to the rolling hash (when you do the multiplication with prime number); you might want to check other resources on rolling hash.

^ problem F Pasti Pas!

Problem H can be solved by Dynamic Programming,here I find two states,one is index of the current question,another is wrong answers used/left so far, and then minimize the answer. But this solution does not work,though sample test case gives correct output. I saw another boolean state in some accepted codes in HUST, which tries both minimizing and maximizing. Can you please explain why another state is needed here?