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Codeforces Round 986 (Div. 2)
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Codeforces Round 986 (Div 2) - Solution Discussion

Shayan

До начала 09:20:49

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Atcoder Beginner Contest 257 Discussion?

Автор iLoveIOI, история, 2 года назад, По-английски

I couldn't find a discussion blog for this contest so here's one.

Does anyone know how to solve Ex?

iLoveIOI
2 года назад
36

Комментарии (31)

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ustaritz

2 года назад, # |

For problem D, my solution with Binary Search + BFS works fine except for 01_random_03.txt. Any hint ?

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harpreet1237

2 года назад, # ^ |

← Rev. 2 →

+16

Did you binary search till 4e9 becauese I was facing same verdict till i realized cost could be 4e9.

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why_I

2 года назад, # ^ |

← Rev. 2 →

My solution also fails on the same test case. Is there anyone who could look into my code.

code

Your code here...

void myfunc()
{

            int n;
            cin >> n;
            vector<vector<int>>vv(n, vector<int>(3));
            for(int i=0; i<n; ++i)
                cin >> vv[i][0] >> vv[i][1] >> vv[i][2];
            
            vector<bool>vis(n);
            function<int(int, int)> dfs = [&](int cur, int s)
            {
                vis[cur] = true;
                int cnt = 1;
                int x = vv[cur][0], y = vv[cur][1], p = vv[cur][2];
               // cout << x << " " << y << endl;
                for(int i=0; i<n; ++i)
                {
                    if(vis[i])continue;
                    int x1 = vv[i][0], y1 = vv[i][1];
                    int dis = abs(x-x1)+abs(y-y1);
                    //cout << "SE " << x1 << " " << y1 << " " << dis << " " << p << endl;
                    if(dis<=s*p)
                    {
                        cnt+=dfs(i, s);
                    }
                }return cnt;
            };
            int ans = 2e15;
            for(int i=0; i<n; ++i)
            {
                int low = 0, high = 1e11;
                int res = 0;
                while(low<=high)
                {
                    int mid = (low+high)/2;
                    fill(all(vis), false);
                    if(dfs(i, mid)==n)
                    {
                        res = mid;
                        high = mid-1;
                    }else low = mid+1;
                }ans = min(ans, res);
            }
            cout << ans << endl;         

}

I have used #define int long long

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thatsramen

2 года назад, # ^ |

Overflow. P can be 10^9 10^11 * 10^9 = 10^20 which is too big even for long long.

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Kartik1508

2 года назад, # ^ |

-6

explain your solution

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bashkort

2 года назад, # ^ |

rude

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ustaritz

2 года назад, # ^ |

With 4e9 it works fine. thanks.

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rgnerdplayer

2 года назад, # |

+12

Submission In problem G why picking the longest transition (by prefix function) works?

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rgnerdplayer

2 года назад, # ^ |

Greedily removing the longest suffix of T that is a prefix of S seems to work as well.

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milind0110

2 года назад, # ^ |

Yeah did so after the contest and it passed but I am unable to prove it why does it work.

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Zain

2 года назад, # ^ |

Because ans[i]<=ans[j] for i<=j because you can always construct a solution for index i from the constructive words of solution j by either deleting some characters of the last word (giving the same ans) or deleting some words then some characters (giving smaller ans).

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sksshivam007

2 года назад, # |

D was a nice variation of Floyd Warshall algorithm

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Dan4Life

2 года назад, # ^ |

BFS is a nice variation of Floyd Warshall?

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harsh__h

2 года назад, # ^ |

← Rev. 3 →

You can also solve it using a slight modification of floyd warshall by defining

$$$ w[i][j] = \frac{|x_i-x_j|+|y_i-y_j|}{p[i]}$$$

and changing + operation with max

code

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Dan4Life

2 года назад, # ^ |

Interesting solution

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SummerSky

2 года назад, # ^ |

I used this idea as well, but I think that binary search solution is also very nice.

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lbntxdy

2 года назад, # |

I have failed on F because of a little mistake in my BFS :)

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Dan4Life

2 года назад, # ^ |

I got WA in one test case in F smh.

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neelakshsh

2 года назад, # |

Problem C? Can anyone mention the approach? Thanks!

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spookywooky

2 года назад, # ^ |

← Rev. 2 →

We can multiply all w[i]=w[i]*2, then check all w[i]+1 and w[i]-1 to be the possible best answer.

Each check can be done in O(logn) by sorting childs and adulds separate, then binary search the number of correct guessed ones for fixed x.

Edit: Or use prefix sums. Atfer sorting w[], we can split w[] at any position, then the correct guessed number is the number of childs left of the split position plus the number of adults right of the split position.

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llc5pg

2 года назад, # ^ |

Keep track of score0 and score1. Use map to iterate the weights and move the border line of guessing 0 vs guessing 1 from left to right. I submitted a simplified version of the code at: https://atcoder.jp/contests/abc257/submissions/32757902

I was unlucky to only solved it one minute after the contest ended.

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Dan4Life

2 года назад, # ^ |

← Rev. 2 →

check for all w[i], then also check for smallest weight-1 and largest weight+1(Solution)

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ninjamayank

2 года назад, # ^ |

just sort the children array and parent array and apply binary to get the ans. also check if we have only children or only adults

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adityagamer

2 года назад, # |

How to solve E?

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aspireboy007

2 года назад, # |

can any one help me with D , I think I miss understood the question . They are saying that : "Takahashi's objective is to become able to choose a starting trampoline such that he can reach any trampoline from the chosen one with some jumps."

some jumps means we can assume that he will be able to jump for each consecutive pair right ?

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adityagamer

2 года назад, # ^ |

No. Read sample test case explanation

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codermaniac

2 года назад, # |

← Rev. 3 →

Hi Everyone, Can anyone help to find the mistake in my code of F? Link to Solution

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Involution

2 года назад, # ^ |

← Rev. 2 →

Consider the case

3 2

0 1

0 3

Just add a line ans = min(ans, st + en + 2); and it should AC.

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codermaniac

2 года назад, # ^ |

Thanks!!!

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adityagamer

2 года назад, # |

← Rev. 3 →

For problem F, my solution is giving runtime error on 27 hidden cases. Why? https://atcoder.jp/contests/abc257/submissions/32791294

UPD: I have done the same thing as editorial except DFS instead of BFS, which I don't think should make any difference

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Spheniscine

2 года назад, # ^ |

← Rev. 2 →

It makes all the difference, because the graph isn't necessarily a tree/forest, so there may be more than one simple path between nodes of different lengths, and DFS doesn't necessarily find the shortest of them

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