Algorithms Thread Episode 8: Tree Basics

4 years ago, # ^ |

+184

For the record, I like being pinged so that I know when people are talking about me. At least, I certainly don't mind.

→ Reply

aman333

4 years ago, # ^ |

+48

SecondThread We have company! Amazing videos, please continue.

→ Reply

Arcane

4 years ago, # |

+104

Wait..... How come this is on the frontpage of CF?

→ Reply

4 years ago, # ^ |

+139

Wait, I have no idea...

→ Reply

HelloWorld

4 years ago, # ^ |

+26

Looks like SecondThread's contribution will become highest on Codeforces shortly :)

→ Reply

NerfThis

4 years ago, # ^ |

+20

probably a combination of post time and up-votes, plus SecondThread is getting more recognition in this community for good contents :)

→ Reply

near-river

4 years ago, # |

Errichto and Thread are changing the learning game istg.

→ Reply

galen_colin

4 years ago, # |

+94

At this point, since everything's already prepared for it, they might as well add your stuff to EDU

→ Reply

pritishn

4 years ago, # |

+198

Spreading CP knowledge!

→ Reply

atharv

4 years ago, # ^ |

+22

What is the problem with Codeforces EDU? I enjoyed all of their lessons so far. They are all doing a great job. No need to compare 3 great resources.

→ Reply

pritishn

4 years ago, # ^ |

I'm not criticizing EDU or saying it's inferior to others.

I'm talking about the quantity of videos/resources or more like time taken to make new educational videos.

→ Reply

Errichto

4 years ago, # ^ |

+341

Fortunately, it's not a competition. It's more like we push one racing car together (because the community gets the sum of all resources).

Also, there are people like Mike or awoo who actually do significantly more, it's just not in form of videos.

→ Reply

WAontest3

4 years ago, # ^ |

+132

Ari just told you not to do that.

→ Reply

pritishn

4 years ago, # ^ |

LOL

→ Reply

VeryGoodVeryGood

4 years ago, # ^ |

+40

Yeah, listen to Ari, no unnecessary tags.

→ Reply

JayShah610

4 years ago, # ^ |

← Rev. 2 →

Don't spam.

→ Reply

aneee

4 years ago, # ^ |

Well said.

→ Reply

Anushk-24

4 years ago, # |

+10

Thanks, we will try our best to learn new and interesting concepts.

→ Reply

CrypticTaiga

4 years ago, # |

+11

Loved the new format ! This helps gain much more confidence in the covered than just watching a tutorial on some topic and not getting custom problems on that topic.

→ Reply

_xiyan

4 years ago, # |

Thanks, that’s exactly what I need.

→ Reply

smax

4 years ago, # |

The gym problems are very nice for reaffirming one's understanding of tree basics!

For problem D, what was the offline solution you had in mind? The text at the top implies that there exists an offline solution that's simpler than the online solution, but the only thing I could think of is some wack dfs + segtree solution where you update the answer for a query whenever you reach one of its endpoints. That seems way too complicated to warrant explicitly disallowing offline solutions lol.

→ Reply

https://www.geeksforgeeks.org/find-farthest-node-from-each-node-in-tree/

4 years ago, # ^ |

+18

Yeah it's definitely harder to do offline than online. I was also considering at a small-to-big + dfs with a treeset/segtree which I'm pretty sure is possible and can be done in n*log^2(n) without too much difficulty. The main thing is that this might show up as a sub-problem that you have to answer online, so it's good to be able to do that, rather than finding some complex way that ACs but doesn't actually use the new topics at all.

→ Reply

arujbansal

4 years ago, # |

+11

I really love your content

→ Reply

kkkkush

4 years ago, # |

Your screencast(s) are really helpful.

→ Reply

sic_mundus_creatus_est

4 years ago, # |

Please give any hints for second problem.

→ Reply

catalystgma

4 years ago, # ^ |

By the way, can you help me out with my implementation? It's giving WA on test 4..

→ Reply

Elegy_No_Programmer

4 years ago, # ^ |

This might help you.

→ Reply

catalystgma

4 years ago, # ^ |

ty, forgot to update max with the old diameter..

→ Reply

codewhisperer

4 years ago, # |

+21

Dear SecondThread,

Thank you for all the hard work you put in daily! Know that it is recognized and greatly appreciated, It’s an honor for our codeforces community to have someone like you.

May you reach every height of success!

→ Reply

doped.silicon

4 years ago, # |

Hey nice video,

In the tree shown at 36:11, how can I find the sum over the path starting at the node with input time 3 and the node with input time 13? I can think of a way to do it but it requires calculating LCA of the endpoints, is there a way to do it without LCA?

→ Reply

4 years ago, # ^ |

+15

Thanks! Correct, if you need to find some path aggregate in which one node isn’t an ancestor of another, you need to split it into two paths using LCA. (Unless there is some other more clever way that I’m not aware of)

→ Reply

Qb1l4l

4 years ago, # |

Can Someone Explain the Samples of Problem F Please

→ Reply

4 years ago, # ^ |

Yeah, maybe part of it isn't super clear: for each query where x = 0, you can consider pairing things however you want just before for that query. These pairings aren't permanent, they are just considerations for that query. The way of making the pairings is to do minimize the sum of distances between every pair of paired seeds/pots.

→ Reply

xodeChemist

4 years ago, # |

expecting more and more from all of you

thanks SecondThread

→ Reply

frost_

4 years ago, # |

The gym-set along with the usual videos format is indeed really nice. Thank you for your hard work.

→ Reply

giant_text

4 years ago, # |

-17

I've just failed my math exam because of secondthread, he says that pi = 3 in 102694A - Circumference of a Tree, so I used pi = 3 to calculate the volume of a sphere and I got 0 marks on that problem. Guys, don't trust secondthread.

→ Reply

4 years ago, # ^ |

+28

Clearly the teacher's solution is wrong then. I have an elegant proof that pi is equal to 3, but it's too long to fit in the margin of this comment thread.

→ Reply

chick_magnet

4 years ago, # ^ |

For people not getting this,its a reference to Fermats theorem

→ Reply

4 years ago, # |

A bit off topic, but can we expect a video on round 1 problems of hackercup, SecondThread ?

→ Reply

KiBie

4 years ago, # ^ |

+10

He said he won't be taking part in hacker cup in one of his streams.

→ Reply

4 years ago, # ^ |

I think...he said something along the lines that he wasn't sure if would be allowed to....but I might be wrong...

Also, I found a comment on youtube...where he answered it so I guess that's it for this thread! (which is a cool line that I am hoping will catch on...lol)

→ Reply

4 years ago, # ^ |

+11

I’m not allowed to compete in hackercup, so probably not from me. I know Errichto has videos of him solving them, and there are some nice solutions on the Hacker cup website, too.

→ Reply

mr_swashbuckler

4 years ago, # ^ |

+13

Why aren't you allowed to compete in hackercup?

→ Reply

I_love_sqrtdecompton

4 years ago, # ^ |

He works at Facebook. Employees are not able to compete, similar to the policy for GCJ

→ Reply

4 years ago, # |

← Rev. 7 →

Hey guys, I am bit stuck on prblem B, my approach was:

start from arbitary node and find the farthest node, call it 'root'
from 'root' , do a bfs and find the distance of all other nodes
The maximum distance now will be the diameter,now,
If distance of some node from 'root' is same as diameter or zero, the answer for this will be diameter+1 , else the answer for this node will be same as diameter

I am getting a wrong answer on test 6. Can someone plz help?

Any general tips to debug will also be appreciated.

→ Reply

4 years ago, # ^ |

There can be multiple "root". Take care of that also

→ Reply

mogamboCodes

4 years ago, # ^ |

I took care for that. I tried DFS from any general node..got all the max distant nodes.. Then ran a dfs from all of them to get their end points...of maxDiam..

Then simply checked if for i is it in the stored points..I added diam+1 as asnwer..else diam is answer.

Getting TLE for TC4 https://codeforces.me/gym/102694/submission/90401537

→ Reply

4 years ago, # ^ |

Make sure you dont linearly check, probably some faster way

Spoiler

→ Reply

mogamboCodes

4 years ago, # ^ |

I didn't get the spoiler hint..It would be great if you could eleborate with some another example which could relate things up.

As I got that searching linearningly for N nodes would difinately be TLE...

→ Reply

4 years ago, # ^ |

Find the boundary points. Think of a better way than a linear search.

Spoiler

→ Reply

mogamboCodes

4 years ago, # ^ |

There is little confusion. I am using Set for finding the elements if the element is end point of diameter. The only linear part I am doing is with find function which calculates the dep and diam initially.

→ Reply

4 years ago, # ^ |

Yeah, that's not the intended solution at all. There shouldn't be any need for a log factor here.

Spoiler

→ Reply

4 years ago, # ^ |

← Rev. 5 →

What if the tree is like this?

Tree

From node 1 you will find 7 (or 8) as the farthest node. And then from 7 (or 8) you will find 1 as the only farthest node. The problem is that your algorithm will consider only 1 and 7 (or 1 and 8) as all the possible nodes which should increase the diameter by 1, but actually all of them will increase it (1, 7, 8). You can try this example yourself for more details.

→ Reply

piyush33patel

4 years ago, # ^ |

Hey Brodicico! Could you(or anyone) please tell me what mistake I am making. (I am continuously getting WA on test 5 and I am unable to figure out what mistake I am making — whether it is the implementation or the logic)

LOGIC :

CODE :

#include <bits/stdc++.h> 
using namespace std; 

#define FileIO freopen("input.txt", "r", stdin);  freopen("output.txt", "w", stdout);  ios_base::sync_with_stdio(0);
#define IO ios_base::sync_with_stdio(0);  cin.tie(0);  cout.tie(0); 
#define ll long long
#define endl "\n"
#define mod 1000000007
#define maxN 300003

vector<int> adj[maxN];
int dis[maxN];

void dfs(int s, int par, int l)
{
    dis[s] = l;
    for(int i : adj[s])
    {
        if(i == par)    continue;
        dfs(i, s, l+1);
    }
}

int main()
{
    int n;
    cin>>n;
    int degree[n+1] = {};
    for(int i = 1; i < n; i++)
    {
        int u, v;
        cin>>u>>v;
        adj[u].push_back(v);
        adj[v].push_back(u); 
        degree[u]++;
        degree[v]++;
    }
    dfs(1,0,0);
    int maxDis = INT_MIN, node = INT_MIN;
    for(int i = 1; i  <= n; i++)
    {
        if(maxDis < dis[i])
        {
            maxDis = dis[i];
            node = i;
        }
    }
    dfs(node,0,0);
    int diameter = INT_MIN;
    for(int i = 1; i <= n; i++)
    {
        diameter = max(diameter, dis[i]);
    }
    for(int i = 1; i <= n; i++)
    {
        if(degree[i]==1 || degree[i]==0)
            cout<<(diameter+1)<<endl;
        else
            cout<<(diameter)<<endl;
    }
    return 0;
}

A counter test case with its correct output is highly appreciated.

→ Reply

4 years ago, # ^ |

Try a case where you have a tree in a chain form like 1-2-3-4-5-6-7-8-9. Then connect node 10 with vertex 5.

The tree will be :

1-2-3-4-5-6-7-8-9
        |
        10

For node 10 it isn't correct to output diam+1.

→ Reply

ab_pakdo

4 years ago, # ^ |

bro i am getting correct answer for this one but still my code fails no 4th test case

→ Reply

4 years ago, # ^ |

Send me your code in private.

→ Reply

4 years ago, # ^ |

There's a hint in the video..watch it again....

→ Reply

avi_dak_hyd

4 years ago, # |

+10

This was a wonderful effort . can anyone give me the link for other episodes.

→ Reply

4 years ago, # ^ |

Here's the playlist.

→ Reply

jinmingli

4 years ago, # |

Your screencast are really helpful. Thank You!

→ Reply

arujbansal

4 years ago, # |

← Rev. 2 →

1006E - Military Problem
Great problem on one of the concepts explained in the video. Highly recommend it!

My solution for reference: 90223865

→ Reply

4 years ago, # |

Someone give me a hint on E, please

→ Reply

4 years ago, # ^ |

+11

I tried to find the topological order and size of each subtree. Then did used segment tree for updating and range queries. But it fails in test 4. Any idea guys?

→ Reply

_runtimeTerror_

4 years ago, # ^ |

Overflow issues .. Even i am not getting how to do that .

→ Reply

4 years ago, # ^ |

Maybe we should divide all node values by 10^6 and then build ST on doubles? If product on subttee is bigger then 1000, print 100000000, else print result

→ Reply

Nachiket

4 years ago, # ^ |

+17

Spoiler

→ Reply

4 years ago, # ^ |

← Rev. 2 →

thank you so much!!! it worked!!! you are amazing

→ Reply

bhagwa_brigade

4 years ago, # ^ |

How did you deal with log and pow function precision issues??

→ Reply

4 years ago, # ^ |

There were no issues for me with pow and log2. Code

→ Reply

Imran_Farid

4 years ago, # |

+10

As a beginner, your episode has really taught me a lot about the many application of trees. Thanks a ton for this!

→ Reply

4 years ago, # |

Hey can someone recommend a good list of problems which contains different( hopefully all theory) types of problems on graphs and/or trees....It would be much appreciated....

→ Reply

Bojack_human

4 years ago, # |

can someone help me , I am getting memory limit exceeded in B. https://paste.ubuntu.com/p/6B8kdrvkyh/

→ Reply

4 years ago, # ^ |

Hey buddy, I saw your code, actually I think you can simplify the structure itself. Also the variable parent can be avoided, vis can be made boolean. Have you checked if the bfs doesn't explodes the queue?

→ Reply

Bojack_human

4 years ago, # ^ |

← Rev. 2 →

I made vis boolean , now i getting tle. thanks man ,Changing the structure now. changed endl to "\n" got accepted.

→ Reply

sambit95

4 years ago, # ^ |

← Rev. 2 →

Didn't know "\n" could change a whole lot of things. Followed some blogs on CF on this. Finally got AC after all the TLEs.

→ Reply

john.pl

4 years ago, # ^ |

The thing is that endl flush the output and that is slow...

→ Reply

4 years ago, # |

← Rev. 4 →

Any idea for D. Till now , this is how I am going: ( I guess m is always n-1).

we need to find the minimum valued edge between sources and destination of the flow network, (as its a tree <or I am wrong here?> and only one path is present between any two nodes).
we can do this by storing values of minimum for 1,2,4,8,16.. jumps above for each node, and find them just like we build the jump array in binary lifting.
Now we can move to the lca and keep calculating the minimum along both souces to lca and destination to lca, again using the binary lift technique.

Am I going the right way ? ( Wrong answer on 6)

→ Reply

https://codeforces.me/blog/entry/63071#:~:text=Sometimes%20using%20endl%20in%20place,and%20flushes%20the%20output%20stream.

4 years ago, # ^ |

+13

I do actually have an idea of how to solve D, yeah. Your general approach seems right, but I’m not going to discuss solution details too in depth for a week or so.

→ Reply

Adson_S

4 years ago, # |

Someone give me a hint on F, please

→ Reply

adhvana

4 years ago, # |

← Rev. 2 →

My 2-dfs calls in B is receiving TLE, is it expected ?

→ Reply

sambit95

4 years ago, # ^ |

Nope! Use "\n" instead of endl, if you're using endl.

→ Reply

adhvana

4 years ago, # ^ |

← Rev. 2 →

I am not using "\n"

→ Reply

sambit95

4 years ago, # ^ |

https://codeforces.me/blog/entry/43780

I faced many TLEs yesterday due to endl. Followed these blogs mentioned above then got the ans.

→ Reply

adhvana

4 years ago, # ^ |

This is not the reason for my TLE verdict

→ Reply

Convergence

4 years ago, # ^ |

I did 2 BFS calls too and it's getting TLE for test case 16. Can't seem to figure out why its TLE tho when at max the complexity would be O(2*(6*10^5)) which is around 10^6. Can anyone help?

→ Reply

Convergence

4 years ago, # ^ |

figured it out now, indeed changing endl to "\n" helped. Thanks!

→ Reply

LightSky

4 years ago, # |

My code for problem B https://ideone.com/dd5gOx with poor English commentary :)

→ Reply

4 years ago, # |

← Rev. 2 →

Math burned me, yet again!!

For E the products can be as large as 10^500000 ( that is ((10^5)^(10^5)) ) and we have to store approx 10^6 of those.....how can it be done....can I get a hint?

→ Reply

Adson_S

4 years ago, # ^ |

you can use logarithm to store the values, and then restore the value with a power of 2

→ Reply

4 years ago, # ^ |

Wouldn't we loose precision? Can you plz elaborate...?

→ Reply

4 years ago, # |

The problems are really, really good. Even if they use basic concepts they were not actually so easy (at least for me). I found the fifth problem very cool and challenging if you don't have experience using mathematical functions (I didn't until this problem :D). I give all my respects to the creator of this problemset. Thanks for investing your time in helping others.

→ Reply

4 years ago, # ^ |

+12

Thanks, I’m glad you enjoyed them :)

→ Reply

ksb_451

4 years ago, # |

For question C the code is running on my computer btt it shows runtime error for test case 3

90309168

→ Reply

yougaindra

4 years ago, # ^ |

since contest is running, nobody can open the link you posted. Also since test case 3 is visible you can directly debug in local environment to find reason for RE.just use gdb/lldb to step through code.

→ Reply

ksb_451

4 years ago, # ^ |

test case 3 is running in my local env just fine. btt it shows runtime error whwn i submit.

→ Reply

FinalBoss_

4 years ago, # |

I haven't registered yet does that mean i'll have to wait for next 12 days?

→ Reply

Harshil_

4 years ago, # ^ |

You can register by going to the gym tab.

→ Reply

Falseeee

4 years ago, # |

Image:

Hello, i am currently at problem E and i dont understand how does it is not 1.5 in the second example, at the second line.

Since it is 1-rooted, 1 has value 3, 2 has value 2 and 3 has value 1 , no ??? what did i missed here ??

→ Reply

4 years ago, # ^ |

In the second sample, none of the nodes are ever updated, so all nodes have a value of 1. (Since the product of 1*1*1... is 1)

→ Reply

Elegy_No_Programmer

4 years ago, # |

Could anyone please help me with problem E?How to handle such big numbers?

→ Reply

gnudgnaoh

4 years ago, # ^ |

+10

this is answered above :

Spoiler

→ Reply

_xiyan

4 years ago, # |

+13

I realy enjoy E, although I've made 17 WA.

→ Reply

iamskp

4 years ago, # |

← Rev. 2 →

Can someone tell me why bfs doesn't work in promlem A to calculate the diameter. It is giving me tle.

→ Reply

JorgeIbanez

4 years ago, # ^ |

It worked for me, maybe because you're using python and the time is a little tight, try reading this blog to speed up the input and the output. If it doesnt work to you, maybe trying with c++ will be a better way, because I see that the few persons that got AC, their execution time is around to 900ms

→ Reply

tanmay157

4 years ago, # |

← Rev. 2 →

Hi, this is my first time solving Tree based problems on CF. I am having trouble understanding how to give the edges so that I can construct a tree. For example in the first question, the test case is

3 2

2 1

How will we construct a tree with 3,2 as edges and which node will they belong to? How will we construct a tree with this? I have seen some other solutions using arrays but i can't understand them.

Leetcode does it like this-

struct TreeNode {
     int val;
     TreeNode *left;
     TreeNode *right;
     TreeNode() : val(0), left(nullptr), right(nullptr) {}
     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

I have previously solved on LeetCode where we had to just write the function so I am very new to this.

→ Reply

scameeer

4 years ago, # ^ |

The one you are constructing in LeetCode is Binary Tree having almost 2 children.

Here you need to use Adjacency list, which can be implemented via vectors in C++. You should read about it.

→ Reply

jalak_patel

4 years ago, # |

Any hint for problem F? (getting wrong answer)

→ Reply

scameeer

4 years ago, # ^ |

Spoiler

→ Reply

Rock2000

4 years ago, # |

← Rev. 4 →

Thank you very much for this effort. I really liked the problems.

→ Reply

4 years ago, # |

I solved F by observing some examples, can somebody give proof for F, why this works?

→ Reply

Qb1l4l

4 years ago, # ^ |

if you supposed a minimal matching where there are only two nodes on each side , and you paired each node with the one in opposite side you'll find that you can minimize more the distances by pairing each node with the one on it's side.

→ Reply

4 years ago, # ^ |

"you can minimize more the distances by pairing each node with the one on its side."

I am looking for why this is always true. Maybe here it is easy to observe.

→ Reply

Hepic_Antony_Skarlatos

4 years ago, # ^ |

← Rev. 2 →

Consider the graph: 1-2, 2-3, 2-4

Node 1 and node 3 have one seed each. Let us consider that there is a pot in the subtree of node 4 (let's call this node A) and a pot in the rest part of the tree above of the node 1 (let's call this node B).

Optimal solution is to match seed-3 with pot-A and seed-1 with pot-B, because the sum of distances is equal to dist(3, 2) + dist(2, A) + dist(1, B). If you try to "cross" the edge (1-2) and match the seeds in a different way, then the sum would be equal to dist(3, 2) + dist(2, 1) + dist(1, B) + dist(1, 2) + dist(2, A) which is larger than before by 2 * dist(1, 2).

As you can see, you can never avoid some distances, so there is no reason to try to match a seed with an other pot when there is already one available in its subtree.

→ Reply

4 years ago, # ^ |

Thanks, That makes sense.

→ Reply

Hepic_Antony_Skarlatos

4 years ago, # ^ |

You are welcome !

→ Reply

ajayNegi

4 years ago, # ^ |

I'm failing to understand the input test cases in problem F, where queries are :

1 6 2
2 5 3
4 3 0

Above, 2 seeds at node 1 and 2 pots at node 6 make 2 valid pairs(path 2->6 goes through edge 3-4). Similarly, 3 seeds at node 2 and 3 pots at 5 make 3 valid pairs. This path goes through edge 3-4. This makes a total of 5 valid pairs.

6 2 2
4 3 0

Similarly, 2 seeds at node 6 and 2 pots at node 2 make 2 valid pairs and this path connecting 2 and 6 also cross the edge 4-3. So, total of 5+2=7 valid pairs. But, why is the answer for that query 3?

I'm missing something here, kindly help!!

Also, it's written The total sum of the distances between every pair must be minimized to keep clean the air. Does this mean something? I didn't get it.

→ Reply

4 years ago, # ^ |

It's not required that seeds should placed in the respective same pots which were created while creating seeds in each query.
In the above example for the last query we can use 2 pots at node 2 for placing seeds from node 1 or 2.

The total sum of the distances between every pair must be minimized to keep clean the air

This means that sum distance travelled by each seed from the source node to node which contains pot should be minimized.

→ Reply

4 years ago, # |

Will all the test cases be made public after the contest ends?

→ Reply

4 years ago, # ^ |

Sure, I can do that. Ping me after if I forget :)

→ Reply

4 years ago, # ^ |

Thanks for the support!

→ Reply

mudit

4 years ago, # ^ |

ping

→ Reply

nirvana_rsc

4 years ago, # ^ |

Thanks for the cool problems!

I couldn't figure out D. So I'm waiting to check other solutions for some insight.

→ Reply

liveoverflow

4 years ago, # ^ |

← Rev. 2 →

please make the test cases public.SecondThread

→ Reply

4 years ago, # ^ |

How do I do that?

→ Reply

Calm_Down

4 years ago, # ^ |

← Rev. 2 →

SecondThread, Can you make the test cases public now?

→ Reply

subhash.yadav

4 years ago, # |

← Rev. 2 →

I am getting TLE in TC 4. I stretched the tree along the diameter and after that if node is added to the end points of diameter then dynamic diameter will increase by 1 otherwise not. My solution is here. Please somebody tell me error in my approach.

→ Reply

blair_

4 years ago, # ^ |

← Rev. 2 →

Your solution can't be opened until the contest ends. But the solution for your problem is answered above. Link to comment

→ Reply

global_optimum

4 years ago, # |

1 sec time limit is pretty rough for java?

→ Reply

4 years ago, # ^ |

For which problem? I think all the judge solutions are in Java, but I can bump it up if it’s too tight.

→ Reply

global_optimum

4 years ago, # ^ |

For B, i feel like my sol would pass with 2 sec limit, but if not that is fine, I can move on

→ Reply

4 years ago, # ^ |

+37

global_optimum you now have 3x the time limit (3s). One might say it's now a big_boy_time_limit.

→ Reply

4 years ago, # |

Can someone please explain me what's wrong in my logic for B ? I seem to be repeatedly failing test case #6.

Logic

→ Reply

jheel4

4 years ago, # ^ |

This will not give you the correct answer.

Spoiler

Let's say you have the following tree:

Your approach would not give diameter + 1 for all the relevant nodes i.e. 1, 6 and 7

→ Reply

4 years ago, # ^ |

Thanks a lot, that helped!

→ Reply

650iq

4 years ago, # |

Can anyone pls provide some test case for problem C? im getting WA on test 4

→ Reply

4 years ago, # ^ |

I am also stuck on test case 4 with problem C

→ Reply

4 years ago, # ^ |

← Rev. 3 →

SecondThread any hints?

→ Reply

silverfish

4 years ago, # ^ |

I'm not sure if it helps, but I recommend you take a look at the cp-algorithms.com lca tutorial if you're stuck.

→ Reply

4 years ago, # ^ |

silverfish No I know about lca and other algorithm to solve it. It is just there must be a corner case that I am missing. Hence wanted some hints

→ Reply

sjay05

4 years ago, # ^ |

← Rev. 3 →

Kush.code Can you link your code?

Most likely your logic for calculating c'th node on the path from a to b is wrong. Key is to use LCA to determine whether c'th node is from path (A -> LCA) or (LCA -> B).

→ Reply

4 years ago, # ^ |

← Rev. 2 →

sjay05 https://ideone.com/A9iHTM I tried but I could not find any issues. can u take a look

→ Reply

4 years ago, # ^ |

← Rev. 3 →

I solved the problem from a bit by the same idea as you. but i was getting Wr4 because the function that lift the node up by c. so, i advise you to check it, this is my submission, i hope it helps you. - wrong submission - Ac submission

→ Reply

FinalBoss_

4 years ago, # ^ |

Draw a tree and make all possible cases for k, that's all you need to do.

→ Reply

4 years ago, # ^ |

FinalBoss_ there is no k in problem c

→ Reply

ksb_451

4 years ago, # |

← Rev. 3 →

In question no C these are the arrays in my programm my code is runnig is fine in my system btt shws runtime error when i submit can this be a memory issue??

int N, D;
int depth[300010];//depth of node
vector<int> adj[300010];
int par[20][300010];//binary lifting table

Inside main

    memset(depth, 0, sizeof(depth));
    for (int i = 0; i < 300010; i++)
    {
        adj[i].clear();
    }
    memset(par, -1, sizeof(par));

→ Reply

nhantran0

4 years ago, # |

SecondThread, what's the constraint of q in D?

→ Reply

4 years ago, # ^ |

Oh, good question. I’m pretty sure it’s 3*10^5. But like, answer each query in sublinear time please and you should be good to go.

→ Reply

_velvet

4 years ago, # |

Thank you very much for the problemset, it's very helpful! I like how it's not just straightforward "implement LCA and be done with it", but includes some modification that forces you to understand the concept behind it.

I have a question about F. It seems that the observation is

Spoiler

Which seems intuitively true, but how to prove it?

→ Reply

4 years ago, # ^ |

← Rev. 2 →

Spoiler

→ Reply

4 years ago, # ^ |

Can i get a hint for problem F.

→ Reply

4 years ago, # ^ |

+12

The problem is strictly linked with the idea of minimizing the sum of distances between seeds and pots, thus the first thing to think is "when is it necessary to cross edge $$$E$$$ when matching a seed with a pot?".

This is not likely a hint, but let's try to see something intuitively in the following cases:

  R -> root.
 / \
P   S

Here you have a pot and a seed in the leaf nodes, thus the result in any edge would be 1 since there's no other matching possible/closer/minimum. This would give us a total sum of 2.

       __R__          -> root.
     /       \
    N1        N2      -> nodes.
  /  \       /  \
2*P  1*S   1*P  2*S   -> amount of seeds/pots.
N3   N4    N5   N6    -> nodes.

Here our options would be:

Match all pots from $$$N3$$$ with all seeds in $$$N6$$$ and the seed in $$$N4$$$ with the pot in $$$N5$$$. This would give us a total distance sum of 4*2 + 4*1 = 12 and the following edge values:

     __R__         -> root.
   3/     \3       -> edge values.
   N1      N2      -> nodes.
 2/  \1  1/  \2    -> edge values.
N3   N4  N5   N6   -> nodes.

Another option would be to match 1 pot from $$$N3$$$ with the seed in $$$N4$$$, the remaining pot from $$$N3$$$ with a seed in $$$N6$$$, the pot from $$$N5$$$ with the remaining seed in $$$N6$$$. This would give us an smaller total distance sum of 2*1 + 4*1 + 2*1 = 8 and the following edge values:

     __R__         -> root.
   1/     \1       -> edge values.
   N1      N2      -> nodes.
 2/  \1  1/  \2    -> edge values.
N3   N4  N5   N6   -> nodes.

Note that the second option is better because we don't need to cross the root edges multiple (extra) times. Thus, matching seeds/pots locally (closer matches) might reduce the total sum. Also, looking at the last example, try to answer the question "when is it necessary to cross edge $$$E$$$ when matching seeds and pots?" thinking locally.

After solving (or if you still can't solve) the problem look at the spoilers on this thread to clear any doubt.

→ Reply

4 years ago, # ^ |

← Rev. 2 →

Thanks bro for your effort, I think i got the key now.

My Idea

UPD: Solved!, Thank you bro.

→ Reply

rishikeshjoshi123

4 years ago, # |

I am not able to solve circumference of tree problem .. also there is no editorial available .. what to do ?

→ Reply

Qb1l4l

4 years ago, # ^ |

+23

Watch SecondThread Trees video ?

→ Reply

jinmingli

4 years ago, # |

It's really helpful! Thank you !

→ Reply

4 years ago, # |

Can some one plz provide a link to their submission for E. I have understood the approach (partially), but having problem to implement it.

→ Reply

https://codeforces.me/blog/entry/81527?#comment-683124

4 years ago, # ^ |

→ Reply

4 years ago, # ^ |

Why are you directing me to my own comment?

→ Reply

Solution using BIT/Fenwick Tree

4 years ago, # ^ |

Click on the link and see the topmost comment on the screen. Its tumaryui's comment.

→ Reply

PetarMihalj

4 years ago, # |

Great video, thank you!

Also, I think a thing worth mentioning is that segment trees can be directly utilized on incomplete rooted binary trees, while this tree -> Euler path -> segment tree approach helps us reduce the complexity when trees aren't necessarily binary.

→ Reply

dgupta0812

4 years ago, # |

← Rev. 2 →

In problem E SecondThread, I got 4 WA's using Fenwick/BIT Tree but as soon as I changed it to Segment Tress, it got accepted.

Solution using Segment Tree

I am unable to understand why it is happening. If anyone has solved this question using fenwick tree, please look at the code and tell where is it going wrong or share your code.

UPD: I got the error. I was constructing the fenwick tree in a wrong manner.

→ Reply

4 years ago, # ^ |

Hmm, my model solution uses a fenwick tree. That’s odd for sure...

→ Reply

dgupta0812

4 years ago, # ^ |

← Rev. 2 →

If that's so, then I must be doing some silly/ gross mistake that I am unable to find. I hope you wouldn't mind to taking a peek at my code unless you're busy.

Anyway, thanks for the clarification.

UPD: I got the error, I was construting the Fenwick Tree in a wrong way.

→ Reply

4 years ago, # |

Nice guide, really appreciated.

For everyone looking for other problems to apply the tree flattening trick, here are some from CSES problem set:

→ Reply

santa_x112

4 years ago, # |

Can someone please let me know why My code fails on test 5(Problem B)

code

→ Reply

4 years ago, # |

← Rev. 3 →

For problem C: Assume tree like following.

Is the answer (2, 1) or (2, 3)?

→ Reply

4 years ago, # ^ |

2, 3

→ Reply

4 years ago, # ^ |

← Rev. 4 →

Can you hint me, what is the wrong here (if you solved the problem). My solution UPD: SOLVED!

→ Reply

DeCodE876

4 years ago, # |

← Rev. 4 →

Can anyone tell me why I am getting the wrong answer for testcase-4 in problem E, or give me any test case? SecondThread

code

→ Reply

4 years ago, # ^ |

It is due to overflow issue. You can see this thread for fixing it

→ Reply

4 years ago, # |

← Rev. 4 →

Can i get hint for problem F.

→ Reply

4 years ago, # ^ |

Being discussed here.

→ Reply

MrRD

4 years ago, # |

Can someone please explain me the way to approach problem F.I have been stuck at it for quite some time now !!

→ Reply

iabhishek15

4 years ago, # ^ |

← Rev. 3 →

-9

→ Reply

akashAgrawal

4 years ago, # |

can anyone tell how to do c

→ Reply

4 years ago, # |

Auto comment: topic has been updated by SecondThread (previous revision, new revision, compare).

→ Reply

pk1210

4 years ago, # |

Really enjoyed thanks.

→ Reply

4 years ago, # |

← Rev. 2 →

SecondThreads java solutions were posted, but if you need A-E in c++: A B C D E F

→ Reply

Prabhu_AGS

4 years ago, # |

← Rev. 2 →

I'm facing a weird error. The same solution is getting accepted in C++14 and getting RE in C++17 for Problem B.

Accepted code

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef pair<ll,ll> ii;
typedef vector<ii> vii;
const int mod = 1e9+7;
const int N = 3e5+1; 

vi edges[N+1];
int dist[N+1],distfromanother[N+1];

void dfs(int s,int p){
    for(auto x : edges[s]){
        if(x==p) continue;
        dist[x] = dist[s]+1;
        dfs(x,s);
    }
}

void dfs2(int s,int p){
    for(auto x : edges[s]){
        if(x==p) continue;
        distfromanother[x] = distfromanother[s]+1;
        dfs2(x,s);
    }
}

main(){
    int n;cin>>n;
    int edge = n-1;
    while(edge--){
        int u,v;cin>>u>>v;
        edges[u].push_back(v);
        edges[v].push_back(u);
    }
    dfs(1,0);
    int farthestnodefrom1,distance=0;
    for(int i=1;i<=n;i++){
        if(distance<dist[i]){
            distance = dist[i];
            farthestnodefrom1 = i;
        }
    }
    memset(dist,0,sizeof(dist));
    dfs(farthestnodefrom1,0);
    int diameter = 0;
    for(int i=1;i<=n;i++) diameter = max(diameter,dist[i]);
    int anothernode;
    for(int i=1;i<=n;i++){
        if(dist[i]==diameter){
            anothernode = i;
            break;
        }
    }
    dfs2(anothernode,0);
    for(int i=1;i<=n;i++){
        int maxd = max(dist[i],distfromanother[i]);
        if(maxd == diameter) cout<<diameter+1<<"\n";
        else cout<<diameter<<"\n";
    }
}

Same code is getting RE in C++17, I tried
1. using memset for dist arrays
2.return 0 for main fn

SecondThread What could be the problem?

→ Reply

4 years ago, # ^ |

That sounds more like a C++ issue to me than anything else. If I had to guess, I would say it's likely a stack-space configuration issue. I think the RTE is probably that your recursion goes too deep on the rope case for instance, which causes you to run out of stack space.

I don't know what differences there would be otherwise, although I was under the impression that your main was always suppose to return 0...

→ Reply

Prabhu_AGS

4 years ago, # ^ |

SecondThread It throws RTE even for n=1, So I don't think it's a stack space issue. Ya main always return 0.

Maybe any C++ experts could help us Errichto

→ Reply

kartik8800

4 years ago, # ^ |

Guess who was right: https://codeforces.me/blog/entry/81527?#comment-682038

→ Reply

june_waves

4 years ago, # ^ |

+10

For n=1 farthestnodefrom1 will not be initialized before the second dfs, which is the cause of RE I think (undefined behavior)

→ Reply

Mokut

4 years ago, # |

SecondThread the problems aren't available in the gym contest

→ Reply