Vector push_backs take too much time (need optimization)

Revision en6, by polosatic, 2024-08-29 13:32:23

Recenlty I was solving this problem. It was obvious strongly connected components. I wrote them and got TL14: 277474774.

I knew that push_backing to 1e6 vectors is slow due to the dynamic reallocation. I know the only way to handle it: precompute the size of each vector, and reinterpret vector<vector> as a static array. It was awful to implement, but I did it and got TL38: 277476691

Then I tried many things like removing the vector of Euler tour, adding pragmas, but have not passes the test 38. So, there are 2 questions for experienced optimizers:

  1. Is there another, simpler way to speed up vector push_backs, without precomputing each vector's size?

  2. Why does my final solution get TL, and how to speed up it? Constraints up to $$$10^6$$$ are enough to pass in this problem I think.

UPD: #define vector basic_string is a useful optimization. EDIT: do not use this define if you use vector<vector<int>> vec instead of vector<int> vec[N].

UPD2: found this blog. There are some explanations.

History

 
 
 
 
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  Rev. Lang. By When Δ Comment
ru6 Russian polosatic 2024-08-29 13:33:55 109
en6 English polosatic 2024-08-29 13:32:23 103
ru5 Russian polosatic 2024-08-29 13:12:24 1 Мелкая правка: 'и в 1e6 веторов очен' -> 'и в 1e6 векторов очен'
ru4 Russian polosatic 2024-08-29 13:10:32 96
en5 English polosatic 2024-08-29 13:09:47 103
ru3 Russian polosatic 2024-08-29 13:05:01 138
en4 English polosatic 2024-08-29 13:03:50 61
en3 English polosatic 2024-08-29 13:01:59 68
en2 English polosatic 2024-08-29 08:09:26 44 Tiny change: 'ion get TL? Constrai' -> 'ion get TL, and how to speed up it? Constrai'
ru2 Russian polosatic 2024-08-29 08:08:37 1279
en1 English polosatic 2024-08-29 08:01:43 902 Initial revision for English translation
ru1 Russian polosatic 2024-08-29 08:01:04 902 Первая редакция (опубликовано)