# FCTRE Video editorial :Along with Mo's Algorithm course

Here is the video editorial for the problem FCTRE

Prerequisites

1. DFS (Graph theory playlist , if you want to study it)
2. Mo’s Algorithm(Mo’s video lectures : Here)
3. Modulo inverse
4. LCA (Learn about LCA : Here)

PS : Sorry for making video too long , I didn’t wanted to but couldn’t help.

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again the same post?

yeah because codechef has deleted my previous post for I do not know what reason

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I tried doing this with the Naive approach of your video on LCA, but getting a WA on SubTask 1. Can you help me out figuring out what is wrong?
https://www.codechef.com/viewsolution/31525318
First I precomputed the Sieve, and added factors and their powers for each weight. Then kept adding factors and powers as I moved from u v to their lca

first thing you are doing wrong is you are swapping u and v based on the nodes values , but you have to swap u and v based on their level in tree (as explained in the video)

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Thanks man, that did it. That was the only problem…

I dont know if it is okay to ask here, but I tried this question many times and had written my solution ( CodeChef: Practical coding for everyone )on the idea of Mo’s algorithm, flattening of tree (tour on tree) but after several attempts I wasn’t able to crack TLE on last few cases. Can somebody help what mistake I might be commiting. Please help.

change you comparator function

``````bool operator < (const query& rhs){
return (BL[l] == BL[rhs.l]) ? (r < rhs.r) : (BL[l] < BL[rhs.l]);
}
``````

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Massive upgrade, initially it was all tle except one of final test case, now it is all passed except one. Thanks man, that was superawesome, great. Only One left CodeChef: Practical coding for everyone
I have to try to optimise something else as well. Let me try that and will let you know.

Thanks a lot again

https://www.codechef.com/viewsolution/31888586

If you don’t mind I have one more question, how that comparator had such great effect and how did you get to know about that

Well i was as fed up as you were because of that TLE , i tried so many optimizations myself but couldn’t go through the last test case , after almost giving up I thought to ask on codeforces about optimization techniques for Mo’s.
Then before asking i thought to do a google search(should have done it earlier) and found 2 articles explaining optimization on Mo’s.
one of them is what I have sent you

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oh nice, I didn’t even thought of that ,such optimization could ever exist.

That’s why we have a lot to learn

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Hey bro cn u help in resolving the runtime error while storing the dfs-order in the array.While I need to find the LCA for the 2 nodes I tried to store the dfs-order in an array but it is giving array index out of range.

This is the link to my code in which I just scanned the inputs…and stored the dfs-order in an array.But this code is getting runtime error due to array out of index range when I submit it in Factor Tree question…I have removed rest of code and have only given the code which causes runtime error.Please help I am stuck from 2 weeks in removing this runtime error.When I try with my test cases it works fine.but when I try to submit it in Factor Tree question it shown runtime error.
I used the DFS-order to calculate the LCA for the 2 nodes.But this is not much of use for now as I have removed all the rest of code and just eft a code which scan all the inputs and just make the dfs-order.

when you are debugging something , try using print statements (best debugging tool for beginners).
The problem is you are not initializing index variable to be 0 in every test case.

@waqar_ahmad224 thnx bro for the help…

you’re always welcome man

Why did u use block size = 1200 ,shoudn’t it be equal to sqrt(2*n) because tree flattening array size gets twice?

I was doing some experiment , yes you can take that size too

Can you explain more about it because if we are using block size of sqrt(2*n) then though it is passing but max time is 7.11 and with bock size of 1200 max time is 5.34 i think that a huge difference!!!