Let's discuss the solutions of all the problems. I would like to know other peoples approaches for the problems. This time I am only able to solve Problem A. What are your views about the problems ?

Here are the problem statements.

# | User | Rating |
---|---|---|

1 | Petr | 3353 |

2 | fateice | 3306 |

3 | Syloviaely | 3274 |

4 | tourist | 3235 |

5 | OO0OOO00O0OOO0O0…O | 3181 |

6 | Um_nik | 3158 |

7 | V--o_o--V | 3148 |

8 | dotorya | 3145 |

9 | LHiC | 3115 |

10 | mnbvmar | 3096 |

# | User | Contrib. |
---|---|---|

1 | tourist | 178 |

2 | rng_58 | 168 |

3 | Petr | 158 |

3 | csacademy | 158 |

5 | lewin | 151 |

6 | Swistakk | 149 |

7 | Errichto | 141 |

8 | matthew99 | 139 |

9 | BledDest | 138 |

9 | adamant | 138 |

9 | ko_osaga | 138 |

Let's discuss the solutions of all the problems. I would like to know other peoples approaches for the problems. This time I am only able to solve Problem A. What are your views about the problems ?

Here are the problem statements.

↑

↓

Codeforces (c) Copyright 2010-2018 Mike Mirzayanov

The only programming contests Web 2.0 platform

Server time: May/21/2018 04:19:51 (d3).

Desktop version, switch to mobile version.

User lists

Name |
---|

Problem A:

len is length of S.

let count(k) be the number of blue bulbs in range [1, k].

then the answer is count(J) — count(I — 1).

count(k) = count(k % len) + (k / len) * count(len)

for k <= len, we can store the answer in a table.

time complexity O(len)

EDIT1: Here is the AC code. I have completely avoided overflows as I find

f(b) ≥ 0 or not cleverly.(Not using log but finding the GP sum inO(log(VAL)) and returning when sum reaches ≥ 10^{18}For Problem B, for given number

N, you need to find the smallest numberbsuch that there exists somexso that . Iteratexfrom 64 to 2 inclusive, and binary search for value ofb.Consider . This will be an non-decreasing function. So binary search for the value of

bthat makes it 0.*Warning: Beware of overflows. See implementation above.

Why is x from [2,18] ? Suppose , for N=1125899906842623 , the value of x should be 50 for b=2. Please correct me if I'm mistaken. I did the same thing as you explained where x iterated from [2,70]. Although I was getting overflow for the large inputs. Can you please elaborate on how to solve it using log.

Why would be

xfrom [2, 18]? You are right. It won't be. It would be from [2, 64].Regarding usage of log:

You know you are in trouble when

b^{x}is very lagre or (b- 1) *Nis very large. How to check whetherx*y≥ 10^{18}? Simplelog(x) +log(y) ≥ 18. So whenever you are in trouble, you know that result is very large. So you can uselog(x) =log(x+ 1).Finally checking whether

f(b) is >or< 0 becomes easy when integer overflow is occurring. You need to check sign of (log(b^{x}- 1) -log(N) -log(b- 1)).we can avoid using log, we can just iterate for all the bases within 10

^{6}upto all powers such that highest power is less than 10^{18}, Now if the base is bigger than 10^{6}then then number n must have less than 3 bits in its representation .Now we just have to check if 1 +x+x^{2}=n. This is equivalent to saying thatn-1 is product of 2 consecutive numbers , which we can easily check . Also if we need to checkx*y>10^{18}, we can check without using log using this :x>= 10^{18}/y."upto all powers such that highest power is less than 10

^{18}?" I don't think so because limit onb^{x}isN* (b- 1) andNitself can be 10^{18}.For product of two no's, yes you are right.

we'll keep adding along the powers to form the possible numbers . hence although the limit on

b^{x}isN* (b- 1) but on the whole number n the limit is 10^{18},we only need to check the highest power less than 10^{18}Yup I don't think you need to use log. I have provided a function below that will generate all the numbers that can be represented by a particular base.

Seems you are right. I have updated the post and attached the code.

Hey, Could you link me to your program?

I'm not sure how to implement the binary search as well as how to handle large numbers.(I did read your other comment,but I'm still not sure)

You can see it here.

I somehow managed to pass small but not large in B, and I had already taken care of overflow but it seems that overflow is the only possible issue.

I coded something like this: http://pastebin.com/8Y33HnTZ

and handle if cnt != bit_length (ie sz) and tt != x. No idea where did I fuck up.

Hey rachit, I used the same technique as yours in contest but is failing for large case Could you please see what is the problem in this code — https://paste.ubuntu.com/23440777/

Yes. I have updated your code. Problem was in expo function. :)

Thanks very much, made very silly mistake :(

for length x, we need to find b satisfy the condition, for N , the most significant bit is b^(x — 1), b is the only one could be result, so, we can get it by b = pow(N, 1.0 / (x — 1)), if b > 1,then we just need to check whether length x of base of b is the ans. i learn it from hdu_toraoh.sorry for my poor english.

Problem B:

just iterator over B and nr_digit is also fine...

trick : use a multiprecision library to deal with large integer, like boost::multiprecision::mpz_int. It's allowed in codejam.

UPD:It's wrong. I didn't remember it clearly until I looked at my code... see comments below.This is what I was doing but it didn't give a solution to a single test case in 3-4 minutes.

I'm wrong. I iterator over nr_digit first, then binary search for B.

So I did the same thing as rachitjain.

link to my code http://ideone.com/pSMiap

many tricks...

Problem C: count the number of tuple (a, b, c, x) s.t.

(1) D divides x

(2) a >= 1, b >= 0, c >= 0

(3) a * x + b * (x + 1) + c * (x + 2) == N

time complexity O(N / D * N), but you have 4min to solve the problem. it's enough...

What would be the solution for the large input?

This is the solution to the large input.

It takes me about 10 seconds to process the large input.

A program that executes 10^11 instructions is acceptable in Google codejam.

I tried implementing what you said but its taking very long.

I think I am doing this part wrong:

`count number of (b, c) by solving indefinite equation`

Could you show me your code or tell me the correct way of counting?

Sorry if its a dumb question.

For this problem, we need to find (b, c) s.t.

`b * (x + 1) + c * (x + 2) = R`

because

`-(x + 3) * (x + 1) + (x + 2) * (x + 2) = 1`

.a solution is

`(b, c) = (R * (-x-3), R * (x + 2))`

all solutions are

`(b, c) = (R * (-x-3) + k * (x + 2), R * (x + 2) - k * (x + 1))`

the number of solutions is the number of k that makes b >= 0 && c >= 0.

This link may be usefull : http://math.stackexchange.com/questions/20717/how-to-find-solutions-of-linear-diophantine-ax-by-c

Generally, we can get initial solution and incremental of it by extended Euclid algorithm.

Thanks a lot! Finally got AC and more importantly, learnt something very useful and important today!

That technique for solving the equation is very useful .

hi, I am a bit late for it though, but I tried implementing it like you said and got correct answer too, but it took like 15 mins on large case input, and furthermore, I checked your submission too and it too took too long to process, is there a simpler solution to it? And isn't the time complexity of your solution like O(n*n*n) roughly?

Here's an O(NT) solution.

iterate through the possible amount of buckets from 1 to floor(N/D)

calculate the amount of balls on the leftmost bucket, this is fixed unless D=1, which could be handled by considering two values for the leftmost bucket.

Note that we only care about the amount of bucket that has the same amount of balls with the leftmost one. This is because the sequence is non-decreasing and maximum difference is two so there is only one partition method for each amount of buckets that shares the same value with the leftmost one.

Code: http://pastebin.com/G5Mmzfdw

It's a very insightful observation.

x is fixed can be proved in another way.

(a + b + c) x <= (a + b + c) x + b + 2 c < (a + b + c)(x + 2)

then

N / (a + b + c) -2 <= x < N / (a + b + c)

so the range of x under fixed (a + b + c) is at most 2. x is fixed if D > 1.

Thanks ! That helped a lot !

I did not look at your code because I want to understand it conceptually first. So basically in addendum to Georeth's formulation, you are fixing

afirst right ? Conceptually how is this fixingbandcor the number of validbandc?This is because I have also fixed [a+b+c] (the total amount of buckets). Therefore if I also fix [a], [b+c] is fixed. That will form a system of two unknowns with two variables, and obviously that will result in only one valid answer set. Also, since the range of [b+c] is continuous, the range of [a] is also continuous and we can make use of this to calculate the range of [a] and the total amount of valid answers in O(1).

Woah! That was a very elegant way of doing this !

Ikr, I wouldn't have thought of this hadn't I misread the statement as solving the question for given N,D and fixed bucket amount. =]

I couldn't solve Problem D, but was trying in the following way.

When P = 2,

Observation:We are given a permutation of 1....N. We need to make it sorted.If array is sorted answer would be

N.Otherwise if answer exists, then there would be 2 sub-arrays [

L_{1},R_{1}] and [L_{2},R_{2}] such thatSubArray[1,L_{1}- 1] will contain all the numbers from 1 uptoL_{1}- 1.SubArray[L_{1},R_{1}] would containR_{1}-L_{1}+ 1 consecutive elements with range [L_{2},L_{2}+R_{1}-L_{1}].SubArray[R_{1}+ 1,L_{2}- 1] would contain consecutive elements with range [R_{2}-L_{2}+L_{1}+ 1,L_{2}- 1].SubArray[L_{2},R_{2}] would contain consecutive elements with range [L_{1},R_{2}-L_{2}+L_{1}].SubArray[R_{2}+ 1,N] would contain consecutive elements with range [L_{2}+R_{1}-L_{1}+ 1,N].I have not considered edge cases where

L_{1}= 1 orR_{2}=Netc. Once I get the correspondingL_{2},R_{2}for givenL_{1},R_{1}answer would bemax(answer, 2 +dp[1][L_{1}- 1] +dp[R_{1}+ 1][L_{2}- 1] +dp[R_{2}+ 1][N]) wheredp[i][j] is the maximum number of partitions of SubArray [i,j] so that when you sort those partitions , SubArray [i,j] also gets sorted.Would be glad if someone could help what to do next.

UPD 1:Here is the code. To check for subarray [i,j] to havej-i+ 1 consecutive elements, you can simply checkmax[i][j] -min[i][j] =j-i.P = 3 is hard.

For the large version of problem B just check for all divisors of (n-1). It worked well for me.