I_love_Captain_America's blog

problemset/problem/543/D
By I_love_Captain_America, history, 9 years ago, In English

http://codeforces.me/problemset/problem/543/D

I did not understand the editorial so I wrote a solution of my own. But I don't understand the tricky cases where it fails. Here is my submission ID 11679268 . My logic is simple:

  1. Do DFS with "1" as root , store results in dp[i]

  2. result[1]=dp[1]

  3. result[i]=(1 + result[parent_of_i]/(dp[i]+1)) * dp[i] % mod;

Where is the logic wrong? I couldn't figure it out.

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9 years ago, # |
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Auto comment: topic has been updated by I_love_Captain_America (previous revision, new revision, compare).

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9 years ago, # |
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This my code http://codeforces.me/contest/543/submission/11369630 — its Accepted

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9 years ago, # |
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To divide result[parent_of_i]/(dp[i]+1) it's better to precalc prefix and suffix multiplications ans just calc something like pref[i-1]*suff[i+1]

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    9 years ago, # ^ |
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    Yes, I calculate inverse power modulo 1000000007 in logarithmic time. I think I found the problem. If result[parent_of_i]==0, then all the further multiplication and division work goes to waste. I tried to work this out, but still haven't got it.

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9 years ago, # |
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Still couldn't fix the bug when (result[parent_of_i]=0 and dp[i]=1000000006) . vintage_Vlad_Makeev can you please explain how prefix and suffix are calculated ?

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