Hello and welcome in Codeforces Round 279 (Div. 2) — description of hacks.

Previous posts can be found here.

Stats

* one of the: INVALID_INPUT, GENERATOR_INCOMPILABLE, GENERATOR_CRASHED, IGNORED, OTHER

Hacks and possible hacks description

490A - Командная олимпиада

The most common errors were:

1. using too small array (or other container), which can result in runtime error

2. wrong usage of min function: for example in C++ such code will compile:

int x = 2, y = 3, z = 4;
cout << min((x,y),z);

and the result would be 3. Why? The min function take two integers — the second one would be z, the first one would be naturally (x, y). What would be result of such thing? Using comma will result in taking the result of the last argument.

More specifically (by Wikipedia): "Comma is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type)."

For another example:

int x = (abs(-4), abs(-3));
cout << x;

returns 3. It won't work without this brackets (think why).

How to correct such thing? It's pretty simple:

min(x,min(y,z))

490B - Очередь

In this problem, there weren't many hacks, most common mistake was simply using too small table.

490C - Взлом шифра

At first, there were no pretests with large n, so big tests with for example answer -1 can grant some points.

Second, the time limit was quite strict for some languages — for example, there were only 1 accept in Python!

Third, some people used strlen function in C++, which has O(n) complexity, so if you use it n times you gain O(n²) solution. Good option is to change it to the string and use method size() instead.

490D - Шоколадки

You may get tle or mle, when you use recursion without memoization. Pretty nice test was:

967458816 967458816
967458816 967458816

967458816 = 2¹⁴·3¹⁰

490E - Восстановление возрастающей последовательности

490F - Турне по Древляндии

Fastest hackers

Congratulations for ShawnDong and dreamoon_love_AA for being first in two different problems!

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Country	#hacks	Hackers
Japan	26	anta [9], uwi [5], atetubou [4], deflat [4], kmjp [2], sugim48 [2]
Russia	26	rtriangle [7], GoDDoS [6], arturom [5], HellKitsune [3], solonkovda [3], SpyCheese [1], Flyrise [1]
India	24	Dr.I_m_possible [5], b4world [4], abhra73 [4], ddt [3], PrashantM [3], achaitanyasai [3], grayhathacker [1], vignesh_m [1]
Uzbekistan	23	Yura_Sultonov [11], monkey_king [8], D05T0N [3], Ozodov__Jamshidbek [1]
Kazakhstan	13	Anuar [5], Tima [3], SmallBoy [3], amiro [1], amanchik [1]
China	11	hq9907 [9], I_Love_Balabala [1], q444372930 [1]
Egypt	11	7asn_a7md [4], ahmedameen [3], Killever [1], Omar [1], Druid [1], mahmoud_arafa [1]