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Educational Codeforces Round 115 Editorial

 2 years ago
source link: https://codeforces.com/blog/entry/95890
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By awoo, history, 2 days ago, translation, In English

1598A - Computer Game

Idea: BledDest

Tutorial
Solution (BledDest)

1598B - Groups

Idea: fcspartakm

Tutorial
Solution (BledDest)

1598C - Delete Two Elements

Idea: fcspartakm

Tutorial
Solution (Neon)

1598D - Training Session

Idea: BledDest

Tutorial
Solution (Neon)

1598E - Staircases

Idea: BledDest

Tutorial
Solution (awoo)

1598F - RBS

Idea: BledDest

Tutorial
Solution (BledDest)

1598G - The Sum of Good Numbers

Idea: BledDest

Tutorial
Solution (Neon)

2 days ago, # |

← Rev. 4 →  

+11

The easiest solution for C(in my opinion): 131432277

P.S. sum * (n-2) = (sum — a[i] — a[j]) * n => we can find for every a[i] all a[j] in O(n log n) using binary search

  • 2 days ago, # ^ |

    If you simplify that equation, you would get (a[i] + a[j]) = (2 * sum) / n, so basically this is just a 2-sum problem where the target value is (2 * sum) / n,

      • 2 days ago, # ^ |

        Can you please check my soln. I have used 2-sum approach but it is giving me WA on test 2. 131563369

        • 22 hours ago, # ^ |

          I also tried the same way and getting wa on 78th in 2tc..LostCoder16 can you pls tell me why it's wa.

          solution: 131653903

          • 21 hour(s) ago, # ^ |

            I havent looked at the correctness of your algorithm but ans = ans + c * d; line has int overflow for sure.

            • 21 hour(s) ago, # ^ |

              ← Rev. 2 →  

              0

              XD. Thanks, for mentioning this. now, after using it long it still showing wa on that tc. Can you please tell me why is it so?

              • 21 hour(s) ago, # ^ |

                ← Rev. 2 →  

                0

                Changing ans = ans + c * d; to ans = 1LL*ans + c * d; doesn't fixes anything. ans is already long long int. Over flow is in multiplication of c*d, change this to ans = ans + c*1LL*d.

                Also ll val = (n * (n - 1)) / 2; has int overflow. Just having val of type long long int doesn't do anything. n and n-1 are still of int type. Change this to n*1LL*(n-1)/2.

                Also, next time just uses C++ 64 bit and long long everywhere.

                Also, if (st.size() == 1) is incorrect fix. You will still get WA on

                Spoiler

                • 20 hours ago, # ^ |

                  Thanks.

                  Got my mistake.

2 days ago, # |

← Rev. 3 →  

+18

sorry i forgot that x is a good number :-_

2 days ago, # |

IDk why but my solution for C gives TLE on Test 17 even though it's the exact same solution as the tester's : https://codeforces.com/contest/1598/submission/131542518

  • 2 days ago, # ^ |

    Don't know why, but using unordered_map gives TLE in this question I have the same problem... use map instead this will solve problem.

2 days ago, # |

Why using unordered_map gives TLE in this question... searching elements using hash map should be faster than the map which takes log(n) time to search instead.
131514464

Also I don't get it... how by adding just these 2 lines which where on a blog,
mp.reserve(1024);
mp.max_load_factor(0.25);
solves the problem of TLE and my soln gets accepted.
131524838

Then I tried a custom hash function rather than the built-in hash function. Which also got accepted. Also the same solution gets accepted in python using dictionaries which also uses hashmap.
Does this mean that the C++ built-in hash function is not good ?
131520347

I tried to find but didn't get some concrete answers when to use map or unordered_map ?
Or using custom hash is better not ?
Please if anyone can help it in.

  • 2 days ago, # ^ |

    There are many hacks on unordered_map and I suppose we should avoid using it in Codeforces contests. Here a blog written by neal showing how it works.

  • 2 days ago, # ^ |

    It's just because c++'s hash function is deterministic so people can hack on this. max_load_factor() basically increase the number of buckets which will break the malicious hacks.

  • 47 hours ago, # ^ |

    As you mentioned that solution with python dictionary got accepted and Thallium54 suggessted in reply that c++'s hash function is deterministic, so I searched for python's and found this here, python uses a hash function which is deterministic within a single run.

    Also the custom hash function which you are using in c++ solution is also deterministic within a single run.

  • 45 hours ago, # ^ |

    Note to self: ALWAYS use custom hash function when dealing with unordered map no matter how unnecesary in a codeforces contest

    • 45 hours ago, # ^ |

      The funny thing in this contest is sjcsjc 's template had custom_hash but still he got hacked for using c++ default hash. 131404435 After that he dropped from 4th rank to 87th rank among rated users.

      • 29 hours ago, # ^ |

        I am a noob.

        • 29 hours ago, # ^ |

          He is a noob.

  • 42 hours ago, # ^ | 

    mp.reserve(1024);
mp.max_load_factor(0.25);

    only changes the test that's required to hack the solution, it doesn't make it unhackable.

2 days ago, # |

← Rev. 4 →  

0

They seemed to be difficult during the contest,but now,I think them is not really tricky.Why I have feelings like this?How can I deal with it?

2 days ago, # |

Nice solution for D!

2 days ago, # |

I suppose that the time complexity of F should be O(n2n+AlogA)O(n2n+Alog⁡A) instead of O(2n+AlogA)O(2n+Alog⁡A). (:

Could someone tell me if there are some mistakes?

  • 31 hour(s) ago, # ^ |

    Can you tell me what is A in this time complexity? I understand where NN and 2N2N comes from but I just can't find anything related to AA.

  • 19 hours ago, # ^ |

    You are right, it's n⋅2nn⋅2n instead of 2n2n.

2 days ago, # |

Any other solution for problem D?

2 days ago, # |

In Problem D, I don't understand why the only bad triples are (xa,ya),(xb,ya),(xa,by). Isn't there a possibility of having three same topics, such (1, 1), (1, 2), (1, 3)? I've been looking at the explanation for so long but still can't figure it out :(

  • 2 days ago, # ^ |

    but their difficulties are different so they aren't bad triples.

    The problem statement says "at least one of the conditions are met"

    • 2 days ago, # ^ |

      Ah, now I get it. I really appreciate your reply. Thanks!!!

2 days ago, # |

← Rev. 3 →  

-9

Spoiler

2 days ago, # |

← Rev. 2 →  

0

For D, can someone explain to me why there can't be a triple of the form [(Xa, Ya) , (Xb , Yb) , (Xa, Yb)] ? EDIT: I figured it out, it's actually the same thing except that the third pair is the central one, instead of the first.

  • 47 hours ago, # ^ |

    Yes, correct. I had the same doubt.

46 hours ago, # |

For 1598C:

No, you don't have to use map in such a hacky way (or in another word, full of patches).

131572801

======================================

Instead of keeping a map of all elements, we keep a map for number of first i items ([0, i-1])

So the core logic can be much more easier:

in C#:

for (int i = 0; i < n; i++){
    // update the answer
    if (cnt.ContainsKey(target - data[i])) answer += cnt[target - data[i]];
    // update the cnt map, for (i + 1) to use.
    if (!cnt.ContainsKey(data[i])) cnt.Add(data[i], 0);
    cnt[data[i]]++;
}

or in C++:

for (int i = 0; i < n; i++){
    // update the answer
    answer += cnt[target - data[i]];
    // update the cnt map, for (i + 1) to use.
    cnt[data[i]]++;
}

44 hours ago, # |

← Rev. 12 →  

0

i tried to add photo for my comment but the comment become empty any one can help me

25 hours ago, # |

In problem D can anyone please explain why we won't be overcounting in the editorial solution?

  • 25 hours ago, # ^ |

    Because he told you in the problem statement that the problems are distinct

15 hours ago, # |

Regarding problem D, I have a question, and I want a simple answer The first is where the values in this equation (n — 1) * (n — 2) / 6 come from. I know it's about knowing the number of ways to choose, but I don't know where the division by six came from and why we subtracted it from one and two

  • 8 hours ago, # ^ |

    You can have an overview of basic combinatorics. It is like NC3,that means you have to select any three out of N so when we apply this , it becomes N!/(3!*(N-3)!) , you can expand N! as N*(N-1)*(N-2)*(N-3)*(N-4)... and so on, you can see that (N-3)! which is in denominator will cancel the numerator from (N-3)*(N-4)*(N-5)........., and that last you will be left with (N*(N-1)*(N-2))/3! and 3!=3*2*1=6, so at last it becomes N*(N-1)*(N-2)/6.

5 hours ago, # |

Alternative solution for 1598D - Training Session. I did see restriction that all problems pairs of parameters are different, but I had no idea how to use this fact. My solution doesn't rely on this fact at all. Solution turned out to be much harder so I wasted a lot of time to implement it during round.

Here is main idea. Think of how to calculate all triples with different topics.

There is easy way

Apply the same idea to problem's difficulties. We count something twice. What to do?

Well

4 hours ago, # |

Detailed description how to implement 1598E - Staircases in O((n+m+q)log(n+m))O((n+m+q)log⁡(n+m)) time and space.

Solution

3 hours ago, # |

A slightly different solution of 1598F - RBS.

Solution

report

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