Welcome after the long break in hacks' describtion in Codeforces Round 270.

This round had really good pretests, so we didn't have many hacks (81 successful ones), but let's have a look at them.

Stats

Description of hacks

472A - Design Tutorial: Learn from Math

There were no particular errors, most of the hacks were just using some easy to check errors: like using 1 as composite number, bad checking if number is composite (forgetting about squares of number, this is not checking last possible divisor ), etc.

Most common testcase was 17. It is 8+9, but many people were checking numbers until floor from (without it).

472B - Design Tutorial: Learn from Life

Again, nothing special in this task. There were only corner cases and bad solutions were caused by some minor mistakes in implementation.

472C - Design Tutorial: Make It Nondeterministic

Nearly all of the hacked solutions were simply too slow and maximum test with large words simply gave "time limit ex.". Also there were some corner cases like no checking first or last word correctly.

472D - Design Tutorial: Inverse the Problem

Hacks in this problem were either really simple (like below)

2
0 1
1 0

or killing slow solutions (many people get tle on this task), common (and sad) mistakes were using set in C++ (which is quite slow) or bad-written dsu.

Fastest hackers

Best hackers

Best rooms

Best countries

Country	#hacks	Hackers
Russia	20	Lelby [3], Nikitosh [2], gridnevvvit [2], VArtem [2], Schullz [2], Copymaster [1], Gassa [1], Guliash [1], Petr [1], ItsLastDay [1], dalex [1], shilov [1], el_sanchez [1], pkhaustov [1]
China	10	sublinekelzrip [2], CNWSYCF [1], dovegx [1], 123qws [1], Flandre_Scarlet [1], dwellings [1], OpalDshawn [1], cbx [1], zhu081607 [1]
India	10	Balajiganapathi [2], JuanMata [2], swap [2], harrypotter192 [1], venki421 [1], noah07 [1], dare1_09 [1]
Japan	8	kmjp [2], ichyo [2], sugim48 [1], logicmachine [1], yaoshimax [1], kzyKT [1]