By Vladosiya, history, 2 weeks ago, 1833A - Musical Puzzle

Tutorial
Solution

1833B - Restore the Weather

Idea: myav, prepared: myav

Tutorial
Solution

1833C - Vlad Building Beautiful Array

Tutorial
Solution

1833D - Flipper

Idea: Gornak40, prepared: Aris

Tutorial
Solution

1833E - Round Dance

Tutorial
Solution

1833F - Ira and Flamenco

Idea: Gornak40, prepared: Gornak40

Tutorial
Solution

1833G - Ksyusha and Chinchilla

Idea: Gornak40, prepared: Gornak40

Tutorial
Solution Tutorial of Codeforces Round 874 (Div. 3) Comments (95)
 » first comment
 » 2 weeks ago, # | ← Rev. 2 →   206652299what's wrong in this code?
•  » » 2 weeks ago, # ^ | ← Rev. 2 →   total_num /= ma[cur_ele]; is wrong.Unlike with addition/multiplication, you can't naively perform "division modulo 1e9+7" with the / operator. Instead you should multiply by the modular inverse of ma[cur_ele] (which is a way to perform division with modulo). To calculate the modular inverse of a number x, you can do x to the power of modulo-2 using a fast exponentiation function. More information can be found online.
 » 2 weeks ago, # | ← Rev. 4 →   Much simpler solution for F just uses sorting and queue. Count the frequency of each element and use sliding window of size $m$ on the sorted frequencies. You can keep track of the product of the frequencies in the window using simple modulo math. If at any time you are holding $m$ consecutive elements (simpler definition of magnificent dance) inside your window, you add this product to the answer.https://codeforces.com/contest/1833/submission/206573660Edit: the submission is fundamentally wrong but is accepted... very weak tests, must be.
•  » » Did the same. (Although didn't have enough time to solve during contest)
•  » » Your solution doesn't work for k=6, m=31 8 16 30 31 32
•  » » » You are right... now that I look at my code I am so surprised it got accept... your case isn't even unusual or anything. It is beyond me why a similar one is not in the official test set. I am remove my submission so to not confuse anyone.
•  » » » 2 weeks ago, # ^ | ← Rev. 3 →   Is this related to the presented algorithm or the implementation?I implemented the same idea in my solution and it works for this case. It also gets AC on the judge. Was this test case added to the tests after the contest?Is there a general reason why this greedy approach with sliding window would fail? My submission is here: https://codeforces.com/contest/1833/submission/206977596
•  » » » » The idea was correct. It helped me understand the editorial's solution. The problem was with the shared code. His sliding window wasn't checking if all the elements inside the window were consecutive.While implementing I was referring to his code and couldn't understand how he was doing consecutive checks. So I tried using test cases that weren't consecutive beforehand and it failed but the same test case got the correct answer when tested on the editorial's solution.About your code. You are checking if the elements between the left and right pointers are consecutive in the else statement of while loop. So you are maintaining consecutiveness.
•  » » » » » It makes sense, thanks for clarifying!
 » A greedy idea for G:Consider a node X that is the parent of m leavesif m>2: The solution does not existif m=2: Two leaves and X form a branchif m=1: Only one leaf (called Y) Move Y up 1 node (set parent of Y is also the parent of X, so that X and Y are siblings)Repeat the process until reaching the root, which can be solved by DFS or BFS
•  » » 207336476 what's wrong in this code? Please.
•  » » » 11 days ago, # ^ | ← Rev. 2 →   Check this test case:41-2 2-3 2-4I assume that you forgot the case when there is only the root left after running DFS
•  » » » » Oh,yes! b（￣▽￣）d　 Thanks
 » An O(n) Solution for D: 206614690
•  » » An O(n) solution which is quite short: Submission
•  » » » Nice
•  » » » An O(n) solution which is even shorter: 206718901
•  » » » » Great! (I like the way you defined "FOR".)
•  » » » » » It's stolen, but thanks
 » 2 weeks ago, # | ← Rev. 2 →   In G, it is sufficient to just cut edges leading to subtrees with a size that is a multiple of 3, which leads to a very clean solution. Of course, some basic check for validity is also needed, such as making sure the correct number of edges were removed and that there is a proper amount of nodes.
•  » » Thanks for the idea! just implemented it : 206740622
 » D, E and F were nice. Thanks.
 » A greedy solution for G which works in O(NlogN):First root the tree(Take 1 to be the root) using dfs. For creating a branch, consider the leaf node at maximum depth. For this I used a priority_queue of pair and then extract the node with maximum depth. For this to be in a branch, this node along with its parent has to be in the branch. Now for the 3rd node, if the parent has another child which is not part of another branch then take it otherwise take the parent of the parent as the third node. Now if the parent of the node at maximum depth has more than 1 other child(apart from the max depth node) then a solution to this problem does not exist i.e. output -1. Also if any of the nodes i.e. either the parent of a node or the case in which parent of parent of node is considered and that node is already part of another branch, then the solution does not exist i.e. output -1. Rest is the implementation and processing part which is there in this solution: 206591154
 » 2 weeks ago, # | ← Rev. 2 →   My simple greedy approach for G using degree and dfs: https://codeforces.com/contest/1833/submission/206667701
 » 2 weeks ago, # | ← Rev. 2 →   For G , I calculated the diameter of the tree and performed a simple dfs from one of the leaf nodes of the diameter , here is the code : 206572278
 » An O(n) solution for D is here: 206665067
 » Here's my O(n) solution to D:
 » On the problem G, I just try to go DFS on a tree, for a vertex u and vertex v is a direct child of u and if v is already searched, I'll check if the size of subtree at vertex v would add to the size of subtree at vertex u so that the size of subtree at u is still less than or equal to 3, if yes, I merge these two vertex in DSU, otherwise the edge connect u and v will be cut. After the DFS, I check all the component in DSU if the size of all of them is equal to 3. But I dont really know how to prove this greedy solution, I just feels like to do it and it got AcceptedThis is my solution: 206610493
•  » » My solution is similar to this idea but did not use dsu just stored the candidate edges to be removedthis is the solution: 206535400
•  » » prod /= c[k];This is incorrect. You should multiply prod by the inverse of c[k] instead and to find the inverse you would need binary exponentiation. Refer this Binary exponentiation.
•  » » » Dude, still got the same problem! :( Here's the submission
•  » » » » Bruh! You must keep all variables in 64bit, int would lead to overflow.
 » Good contest
 » 2 weeks ago, # | ← Rev. 2 →   In the editorial of problem F, the definition of prefix product should be $p_i=c_{1}\cdot c_{2}\dots c_{i}$ or $p_i=c_{i}\cdot p_{i-1}$, Please fix it, thanks.I have another question about this part: if $p_{i}=0$, $p_{i+m}=0$ at the same time, but there is actually no zero in the interval $[i,i+m]$, this way of calculation may be wrong.Example: $n=6$, $m=3$, $c=[1,0,1,1,2,1]$, now $i=3$, we found that $p_3=0$ and $p_6=0$ but actually $c_3\cdot c_4\cdot c_5\cdot c_6=2$, so this is incorrect.If there are other special properties that make this never happen please point it out to me, thank you.
•  » » 2 weeks ago, # ^ | ← Rev. 2 →   Since $b$ (the unique and sorted array) and $c$ (their frequencies in $a$) is derived from $a$ (the original array), there is no way of having a number occurring in $a$ and having frequency $0$.
 » Finally became cyan :)
•  » » :/ not anymore
•  » » » 4 days ago, # ^ | ← Rev. 2 → 0 .ComplaintFrame { display: inline-block; position: absolute; top: 0; right: -1.4em; } .ComplaintFrame a { text-decoration: none; color: #ff8c00; opacity: 0.5; } .ComplaintFrame a:hover { opacity: 1; } ._ComplaintFrame_popup p, ._ComplaintFrame_popup button { margin-top: 1rem; } ._ComplaintFrame_popup input[type=submit] { padding: 0.25rem 2rem; } ._ComplaintFrame_popup ul { margin-top: 1em !important; margin-bottom: 1em !important; } Ab bol... Ab bol na mc
 » 2 weeks ago, # | ← Rev. 2 →   How is the output for the 4-th test in problem D the maximal permutation you can achieve? isn't the correct answer when l=2 and r=6?yeah i understood no need to reply
 » 206709921 Memory limit exceeded G submission with dfs recurssive, greedily checking for 2 children for a cut to be made Can someone suggest how can I solve this
•  » » Try using an explicit stack/queue instead of recursion on the call stack?
 » Thank you for setting this contest for us. I would like to suggest that the description for problem E would be clearer if you include the statement that "Each person participated in exactly one round dance".If this statement were not true (ie each person can participate in multiple dances and remembers one neighbor from one of their dances), then the problem description would still fit (but I believe it's unsolvable)
 » In the C question solution what does .split() do?def solve(): n = int(input()) a = [int(x) for x in input().split()] a.sort() if a % 2 == 1: print("YES") return for i in range(n): if a[i] % 2 == 1: print("NO") return print("YES")t = int(input()) for _ in range(t): solve()
•  » » It splits a string into a list of tokens separated by whitespace. More info can be found here.
•  » » » Got it thanks
 » I made a video editorial for this contest's first 5 problems, i.e. A to E. It has Hindi commentary so if you know Hindi then you can consider watching this video. Video Editorial Link — https://youtu.be/rXj5okWOy4kPlease give your valuable feedbacks in the comment section :)
•  » » give comment in hindi too
 » 2 weeks ago, # | ← Rev. 2 →   .
 » For problem E, I believe the solution's usage of "vector neighbours(n);" isn't necessary.
 » 2 weeks ago, # | ← Rev. 2 →   Honestly I don't really understand what's the purpose of proposing something like problem D in division 3, which I think should be more educational, since beginner coders are the target audience here. What this problem is suppose to teach? Really anything, it's just heavy implementation and corner cases. I got the O(n) solution by myself something like 30 min after the contest and implemented the day after, just for the sake of doing it, being aware this is not worth from the educational point of view.
•  » » I wouldn't consider 30 lines solution heavy implementation... and if you came up with a more implementation-ugly solution (which seems to be in your case) than the model solution, then that alone teaches you that you can come up with a more elegant solution, so what is the issue here?
•  » » » What I meant is that my main goal so far Is learning new things and, implementation apart, this problem does not teach anything at all. This is not exactly the kind of problem I would expect in a division 3. Am I wrong stating this?
•  » » » » 2 weeks ago, # ^ | ← Rev. 3 →   It teaches you how to think about and handle corner cases :)Also, it is not "implementation heavy" at all, about 15 lines of Python.
 » Does someone understand the solution for problem E in the editorial?? My approach was that each cycle greater than 2 had to be a separate dancing group so the last and the first person could connect. In the editorial code, it ignores these cycles of length > 2 now that it only takes in account the leafs(nodes with degree==1) in the graph. It only counts cycles when there are no leafs. Probably I have not seen an observation or something.Can someone please explain me this solution:)?
•  » » If in a particular component every node has exactly two neighbors, it signifies the formation of a cycle. In this case, you should consider it as a round dance. Conversely, if at least one node only has a single neighbor (referred to as a bamboo in the editorial), this indicates two possible outcomes: you can either close a cycle by treating this deficiency as a wildcard, or you can link it to other bamboos to create a larger round dance. This is how I understand it, and I hope my interpretation is helpful.
•  » » » I understand why there is a cycle when in a component every node has exactly 2 neighbours. But let's say I have 1->2->3->4->2. In this case the code will mark it as a bamboo, and in my intuition, this should be marked as a separate dance now that there is a cycle greater than 2. I thought it separated in 2. cycle : 2->3->4->2 AND bamboo : 1. But in the code I see the code only counting the node with degree equal to 1. Maybe I am really confused, not sure This is the code from the editorial that I am talking about. It seems weird that it only cares about the nodes with degree equal to 1 bool bamboo = false; for (int j: component) { if (d[j] == 1) { bamboo = true; break; } }If someone could explain this to me, I would appreciate it.
•  » » » » This scenario isn't possible because '2' represents a person who is part of a round dance and, as such, can only have a maximum of two neighbors.
•  » » » » » Thanks, you're right. It even says it in the comments, I guess I mis read it.
•  » » ummm, you can think it in a better approach.... we will store the the graph in a adjacency list. Then we can just run a dfs for every unvisited node(1-n) and if it gets visited we will mark it as visited. So the number of times the dfs has run is the max count, and while running the dfs we will also check if the graph has cycle, if it has cycle we will increase minimum value by 1, and at last we will just check if maximum is greater than minimum, if it is then we can increase minimum by 1, because the total maximum count will make another cycle. You can see the solution to get a clear idea 207292218
 » 2 weeks ago, # | ← Rev. 2 →   Hi, can someone help me with D question For this input: 1 12 2 3 4 5 6 7 8 9 10 11 12 1 4 why is answer output 12 1 11 10 9 8 7 6 5 4 3 2 Shouldnt we want to move the 9 to the first position? By doing so we ensure that our final array will start with a 9 and will be lexicographically maximum?instead answer moves 12 to start which makes number begin with a 1 then a 2 which would be less than a 9??Please help I dont understand
•  » » 2 weeks ago, # ^ | ← Rev. 2 →   According to the problem, a permutation 'a' is considered lexicographically greater than permutation 'b' if there's an 'i' (where 1≤i≤n) such that a[j] equals b[j] for 1≤j
•  » » » oh got it, thanks
 » Oh man, got super close on F, just needed to debug my prefix product code when the timer ran out
 » I can't understand the meaning of problem E, can someone translate it into formal statement? Thank you very much!
•  » » Given an undirected graph where each node has a maximum of two neighbors, find the maximal and minimal number of connected components the graph can have while still fulfilling the initial condition (each node must have a maximum of two neighbors).
•  » » » Very clear, thank you！
 » Hello, I am newbie here Can anyone tell me why my code falling 206856992. The problem is C.
•  » » what's the meaning of "even" and "odd" in your code? it's too complicated.
 » What does "bamboos" mean in the tutorial for problem E?
•  » » I would call them also "chains", basically just a sequence of connected nodes a — b — c — d ...
 » nice contest
 » A simpler solution of G is 206896718
•  » » Thanks! Now I can understand.
 » I am getting TLE exceeded on testcase 6 in question no. 2. While i think it is in the time limit. Can anyone help me out please? https://codeforces.com/contest/1833/submission/206588962
•  » » The main problem with your code is this line. while(cnt[ind]) ind++; This loop runs for some test cases, like the following: Spoiler1 10 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 
 » I don't understand the 3rd method/solution for problem F. I am talking about this part:"We will use the idea of building a queue on two stacks, but we will support prefix products modulo in these stacks. Time complexity is O(n) and we don't use that the module is prime."I would appreciate if anybody could explain this part in more detail.
 » Could you please explain examples in problem D: 4 4 3 2 1 why can't we just reverse segment with length 1 (without changing anything), and use empty prefix and suffix?or 5 2 3 1 5 4 why can't we invert the first two elements [2,3] so it is 3 2 1 5 4 and then swap empty prefix and suffix [5,4] so it is 5 4 3 2 1 ?feeling really stuck here, please help
•  » » I believe you're getting the 2nd step wrong. You can't arbitrarily choose prefix and suffix after reversing a segment. If you choose to reverse the segment p[l,r] then you have to choose segments p[1,l-1] and p[r+1,n] to swap. In the example you provided with the array [2,3,1,5,4], if you choose to reverse the segment [2,3], you should also swap the segments [ ] (empty segment before 2) and [1,5,4] (segment after 3). Following these steps, the resulting array would be [1,5,4,3,2].
•  » » » oh, i finally got it, thanks a lot!
 » Hey!I'm trying to find a test case that makes my submission give WA on problem G.Currently, I'm getting WA on TC 158:Here is my submission: https://codeforces.com/contest/1833/submission/207332694
•  » » Try this one. Test case1 9 1 2 2 3 1 4 3 5 4 6 2 7 5 8 5 9 
•  » » » Thank you!I got AC. The main issue is that I was tracking the leaves wrong. I was not accounting the degree of the nodes that I've was pushing to the queue.I ended up also doing a random input generator :/, because I just checked this comment.
»

I did the similar code for problem E of this, but I got TLE. The only difference is I applied DFS instead of BFS in my code.

# define S second

using namespace std; typedef long long ll; typedef vector vi; typedef pair<int, int> pi;

bool dfs(int node,vector&visit,vector&parent,vector<unordered_set> adj){ visit[node]=true; bool cycle=false; for(auto i:adj[node]){ if(!visit[i]){ parent[i]=node; cycle=dfs(i,visit,parent,adj); } else if(i!=parent[node])cycle=true; } return cycle; }

int main() { int t; cin >> t; while (t--) { ll n; cin >> n; vector<unordered_set> adj(n+1); vector a(n); for (int i = 0; i < n; ++i){ int num; cin>>num; adj[num].insert(i+1); adj[i+1].insert(num); } vector visit(n+1,false); vector parent(n+1,-1);

int countCycles=0,countNormal=0;
for(int i=1;i<=n;++i){
if(!visit[i]){
else ++countNormal;
}
}
if(countNormal)cout<<countCycles+1<<' ';
else cout<<countCycles<<' ';
cout<<countCycles+countNormal<<endl;
}
return 0;

}

 » 10 days ago, # | ← Rev. 2 →   In Python, the following code TLE's for F. If I uncomment the sorting of the list it passes. What's going on? Am I hitting some bad case with the set() function? def solve(n_orig, m, a): #a.sort() # this is somehow necessary? cnt = Counter(a) sk = sorted(list(set(a))) n = len(sk) #... correct code after that # 
 » in problem B if in array b the minimum element is 11 for array a = [1,3,3,5,9] then is sorting array b efficient?
 » Can anybody explain stack and queue sulution of problem F with more detail?I am unable to understand the idea behind the code
 » why write tutorial when you fail to clear the concept
 » shitty editorial... can't understand a thing
 » This is shit