Welcome in hacks' statistics in Codeforces Round 278 (Div. 2). That was really nice round for hackers! Many possibilities, many hacks, let's have a look!
Post for Div. 1. contest will be added later (there are pretty many hacks to parse :)) is here.
Previous posts can be found here.
Your comments are always welcome.
Stats
| Problem | Successful hacks | Unsuccessful hacks | Other* | Sum | Solutions which can be hacked | Accepted solutions | All solutions on final tests |
|---|---|---|---|---|---|---|---|
| 488A - Giga Tower | 99 (46.05%) | 86 (40.00%) | 30 (13.95%) | 215 | 228 (12.20%) | 1641 (87.80%) | 1869 |
| 488B - Candy Boxes | 23 (50.00%) | 17 (36.96%) | 6 (13.04%) | 46 | 265 (63.55%) | 152 (36.45%) | 417 |
| 488C - Fight the Monster | 21 (65.62%) | 10 (31.25%) | 1 (3.12%) | 32 | 149 (70.28%) | 63 (29.72%) | 212 |
| 488D - Strip | 0 | 0 | 0 | 0 | 16 (76.19%) | 5 (23.81%) | 21 |
| 488E - Prefix Product Sequence | 0 | 0 | 0 | 0 | 3 (75.00%) | 1 (25.00%) | 4 |
* one of the: INVALID_INPUT, GENERATOR_INCOMPILABLE, GENERATOR_CRASHED, IGNORED, OTHER
Hacks and possible hacks description
488A - Giga Tower
The mistake was pretty easy and pretty nice. Many people assumed that we need to step at most 10 steps. The counterexample is simple something near 0: -5, -6, -7 (we need to reach 8, of course). Simple and really effective. It was checked by the test #10.
488B - Candy Boxes
I am not a big fan of such problems: you know how to deal with it, but you have to fight with all corner cases. So what tests we didn't found on the pretests?
First one is simple:
4
1 3 2 2
Some people forgot about sorting numbers before taking range. So if we simple take range as x3 - x0, we have incorrect answer.
Second one is for example:
3
4 8 9
Looking at tutorial's first solution is the second testcase in n = 3 (the answer is 3).
Some people also had some bad outputs (additional numbers, missing numbers in some cases, etc.).
488C - Fight the Monster
Compared to 460 hacks in Div. 1. only 21 hacks in this division is nearly nothing.
Let's have a look at posibilities:
At first, let's look at SashaMN's comment (I have to do the screenshot, because it works much better with his avatar).
Interesting fact is that he hacked successfully 15 times, but his solution didn't pass! Where does it bring him?
Congratulation!
Also, you may need more than 1000HP (what was assumed by many people), so we can hack them with test by eduardische:
1 10 1
99 100 1
1 100 100
and when they use greater number, we can hack them again, with test by Ximera:
1 2 1
99 100 1
1 100 100
Poor people :)
488D - Strip
There were no hacks in this division. What can we hack?
There were some solutions which can get tle (with solution 
or 
). It was possible to get acc with such complexity, but the time limit was quite tight (1 second).
Also some people have problems with negative answers. For example, we have tests #31 and #32:
2 1000000000 2
-1000000000 1000000000
10 3 3
1 1 1 1 1 5 6 7 8 9
The answer is simply -1, but some people did find the answer.
488E - Prefix Product Sequence
There were no hacks in this division. Again, there were trick cases:
Test #29 (one and only composite number with answer YES):
4
with answer:
1 3 2 4
or test #26:
1
with answer:
1
Fastest hackers
| Problem | Time | Hacker | Defender | Hack |
|---|---|---|---|---|
| 488A - Giga Tower | 00:46:39 | rsFalse | day_and_night | 124793 |
| 488B - Candy Boxes | 01:52:08 | El_Ectric | ciocan_catalin | 125426 |
| 488C - Fight the Monster | 01:10:47 | Hactory | NeNikita | 124988 |
Best hackers
| Hacker | Stats | Successful hacks | Unsuccessful hacks |
|---|---|---|---|
| VeniVidiVici | +8-1 (750) | A: 124896 124947 124958 124967 124970 125028 125571 B: 125681 | B: 125662 |
| Shavkat_Aminov | +5-0 (500) | A: 125421 125430 125445 125458 125488 | |
| shank_punk | +5-0 (500) | A: 125294 125454 125462 125483 125578 | |
| Hactory | +5-1 (450) | C: 124988 125105 125291 125554 125581 | C: 125585 |
| iBlackShadow | +4-0 (400) | A: 125440 125461 C: 125558 125725 | |
| dwoolley3 | +4-0 (400) | A: 125335 125383 125396 125433 | |
| Jamol_Mattiev | +4-0 (400) | A: 124987 125023 125422 125485 | |
| allllekssssa | +4-0 (400) | A: 125540 125555 125584 125596 | |
| 404Error | +4-1 (350) | A: 125292 125298 125365 125650 | B: 125715 |
| Lo_R_D | +4-1 (350) | C: 125264 125273 125416 125639 | B: 125619 |
Best rooms
| Room | #hacks | Hackers |
|---|---|---|
| 87 | 9 | VeniVidiVici [8], ankdroid [1] |
| 20 | 7 | mosiomohsen [2], ucsfww646 [1], shirakia [1], k03 [1], nghiand [1], alohamaloha [1] |
| 8 | 6 | Shavkat_Aminov [5], ashkan_d13 [1] |
| 15 | 5 | shank_punk [5] |
| 49 | 5 | Hactory [5] |
| 79 | 5 | nemzs [3], alex-kozinov [2] |
| 95 | 5 | I_Love_Balabala [4], TheDeliriousWave [1] |
Best countries
| Country | #hacks | Hackers |
|---|---|---|
 Uzbekistan | 17 | Shavkat_Aminov [5], Jamol_Mattiev [4], Isfandiyor [3], Jumaboyev_Davlatmurod [2], Yura_Sultonov [1], Sirojiddin [1], Eldor_Zoirov [1] |
 India | 14 | shank_punk [5], aksam [3], vishwacs111 [3], theab [1], shreeshaprabhu [1], ABHISHEK004 [1] |
 United States (USA) | 8 | dwoolley3 [4], El_Ectric [4] |
 Kazakhstan | 6 | AdilbekZh [3], Xephy [3] |
 Iran | 6 | Majid [2], mosiomohsen [2], ATofighi [1], ashkan_d13 [1] |
 Belarus | 5 | nemzs [3], volverine [2] |
Test #29 (one and only complex number with answer YES)
Such numbers are called composite, complex numbers is a bit different concept. Looking forward to the div 1 stats, that's where the real fun was!
Yeah, sorry for that — I am still bad in English. Thanks!