Thus, the only other possible way to color the graph with two colours is if we flip the colors in the subtree of $$uv$$$. output. :D. No, actually, he was teaching and I made up this name and said it, and since then every time haas wanted to talk about this order, he used it. Consider a directed graph given in below, DFS of the below graph is 1 2 4 6 3 5 7 8. Each algorithm has complexities < O(f(n)), O(g(n)) > , it means that this algorithm's preprocess is O(f(n)) and answering a query is O(g(n)) . It is like DFS order, but every time we enter a vertex, we write it's number down (even when we come from a child to this node in DFS). You can visit each vertex several times. For this offline algorithm, the set P must be specified in advance. Next n - 1 lines contains the edges of the tree in the format "xi yi" (without the quotes) (1 ≤ xi, yi ≤ n, xi ≠ yi), where xi and yi are the vertices of the tree, connected by an edge. section of Observation 1, I think that there will be the following correction. Example: Having a rooted tree, each vertex has a value (initially 0), each query gives you numbers v and u (v is an ancestor of u) and asks you to increase the value of all vertices in the path from u to v by 1. But you should try it once. I would love to see your list. I think the low[v] = min(low[v], low[to]) should hold good for the bridges but not for the articulation points. However it was accepted after removing the visited array. If par is an allocated array, (par + 1) is just a pointer operation to one position further from the beginning of the array, in other words, it will be a memory area that is not NULL and therefore will always be true. Codeforces Round #661 (Div. Suppose there is a directed edge $$u \to v$$$, and the depth-first traversal reaches $$u$$$, but hasn't yet explored $$v$$$. Yes, I find the same issue. do you have any mathematical proof for spfa algorithm? Weights Division (easy version) Problem solution dfs tree traversal + greedy + priority queue, Programmer Sought, the best programmer technical posts sharing site. A back-edge $$uv$$$is a solution if and only if $$uv$$$ is the only contradictory edge. Both are useful (C++11). I have another question if you don't mind, how would we get the list of articulation points? Leaderboard Descriptions: System Crawler 2021-01-04; Moses 2020-08-16 joylintp 2019-05-21 qazwsxedcrfvtgby 2018-03-09 674902997 2018-03-07 Tree, Back, Edge and Cross Edges in DFS of Graph. Thanks for the description, I realized it after posting it. Anyone can suggest me a problem, where I've to detect all the nodes part of the negative cycle? It's said that at first Chinese coders used it in programming contests. 06, Feb 19. I believe DarthKnight forgot to mention that he meant the longest simple path problem. This can be trivially computed in time and space. Think of a graph which is a cycle. Must use this as resources if I continue my teaching in Hanoi CS Olympiad team next year. Also it will be nice to see MaxFlowMinCost tutorial here. Its code looks like the combination of Dijkstra and BFS : MST = Minimum Spanning Tree :) (if you don't know what it is, google it). Here is an implementation (and here is the classical one for reference). The sad part is that I can only give you upvote for this amazing article. Anyway here is the first step on how to find an articulation point. The other span-edges still must have a white vertex on one end and a black vertex on the other in any bipartite coloring. Let $$u$$$be a vertex and $$p$$$ be its parent. BFS tree is a rooted tree that is built like this : The most useful and fast-coding algorithm for finding SCCs is Kosaraju. This section is purely an implementation trick. You can read all about maximum flow here. I made a submission to showcase it: 56639790, components_dfs function. I guess there is a typo while writing prim's algorithm. (Notice that according to this, the leaves of the DFS-tree are not articulation points.) In the code you link, I also use it as a marker of "have I already visited this vertex": if lvl[u] is 0, then it is unvisited by the DFS. Can you please explain how is it related to Eulerian path/Eulerian tour/Eulerian cycle? Consider a non-root vertex $$v$$$; let $$u$$$ be its parent in the spanning tree. Note that the length of a block is , which is quite small. Instead of checking if any one of the subtrees have a back edge passing over u -> v, we should check if all subtrees of that vertex has a backedge. Roman planted a tree consisting of n vertices. These are implementation details, and in my opinion this isn't even the most intuitive way to implement finding bridges. As Dijkstra you can use std :: priority_queue instead of std :: set. So this problem is NP-hard. C++ example (it's a little complicated) : LCA of two vertices in a rooted tree, is their lowest common ancestor. This is 19E - Fairy. Now, to answer RMQA(i, j) we have two cases: Six) Tarjan's algorithm O(na(n)) (a(n) is the inverse ackermann function). Indeed, on a practice problem I submitted to, "classical" got 200ms while mine took 280ms. Does it work when parallel edges are allowed? The code below works properly because the lemma above (first lemma): give each back-edge an unique index starting from $$N + 1$$$; for each vertex $$u$$$, calculate the index of the back-edge $$u$$$is under; call that $$\mathrm{cycleId}[u]$$$; if $$u$$$isn't in a cycle then $$\mathrm{cycleId}[u] = u$$$; form a new adjacency list where for each $$u$$$, each instance of $$u$$$ is replaced by $$\mathrm{cycleId}[u]$$$. Here, we will cut the tree into (H = height of the tree), starting from 0, k - th of them contains all vertices with h in interval . You can use tags to find related problems. If an edge u->v is going downwards, u must be in the subtree of v, which is a contradiction. I understood the concept of DSU (in Kruskal) but could not have any clues about DSU on tree (in this post and http://www.codeforces.com/blog/entry/44351). https://github.com/Errichto/youtube/wiki/Learning-resources. :D Thanks :D, Then all your class CALLED it Euler order. 25, Apr 18. Span-edges form a rooted spanning tree, directed away from the root. It remains now to show how the in-block queries can be made. -is-this-fft- I feel dumb asking this. Let me clear, what do you mean by dp[node]? Programming competitions and contests, programming community. Observation 2. Then, we start from the vertex with the greatest finishing time, and for each vertex v that is not yet in any SCC, do : for each u that v is reachable by u and u is not yet in any SCC, put it in the SCC of vertex v. The code is quite simple. If we overshoot the LCA, then we could have reached it anyway due to the special cycle property of the graphs, Thankyou, was struggling a lot in understanding low[u] in typical method, this method is very easy to understand :p. can someone give pseudocode/code to create a dfs spanning tree and to check for articulation points? if the graph has no non-bipartite connected components, then removing any edge will produce a bipartite graph; if the graph has multiple non-bipartite connected components, then it is not possible to make the graph bipartite. Now, we preprocess A' using the Sparse Table algorithm described in lecture 1. In the original problem, the graph need not be connected. However, I did not think that the parent can also be the articulation point, as it is also the part of the bridge. In conclusion, if any vertex has even 1 subtree without any backedge, its an articulation point. Well no, because you still need a way to track whether a back-edge goes "up" or "down". Also, would you care to explain what is the logical meaning of lvl? It's possible to further divide the back-edges into up-edges and down-edges based on their direction. You can prove this algorithm using induction. I talked about SQRT decomposition in the first lecture. I have edited it now. There is no official tutorial, but an unofficial tutorial mentions using complicated data structures like Link/cut tree. It felt like many tutorials didn't really explain how it works, kind of just mentioned it in passing and quickly just moved on to implementation. Let's consider the DFS tree of the graph. memory limit per test. This way we have moved at least one step towards the root. The only way a back-edge can connect these components is if it connects a descendant of $$uv$$$ with an ancestor of $$uv$$$. Direct all of its edges so that the resulting digraph is strongly connected, or declare that this is impossible. Because the graph contains no bridges, there must be a back-edge passing over $$uv$$$: it connects a descendant of $$v$$$to an ancestor of $$u$$$. No.1 707D - Persistent Bookcase . If there is a back-edge between these two components, then the graph is still connected, otherwise $$uv$$$is a bridge. Basically, dp[node] stores the answer for the direct edge coming to the node from its parent. Unlike graph, tree does not contain cycle and always connected. To solve this restricted version of the problem we need to partition A into blocks of size . The root of the tree … MakeSet(u) removes u to a singleton set, Find(u) returns the standard representative of the set containing u, and Union(u, v) merges the set containing u with the set containing v. TarjanOLCA(r) is first called on the root r. Each node is initially white, and is colored black after it and all its children have been visited. it is possible to reach every vertex from the vertex 1. edges that connect an ancestor with a descendant: if the traversal doesn't go from $$u$$$ to $$v$$$, it means that the traversal already visited $$v$$$ during exploring some of the other children of $$u$$$and thus $$u \to v$$$ is a back-edge; if the traversal does go from $$u$$$to $$v$$$, then $$u \to v$$$by definition is a span-edge. Also, I believe that the answer will be the same for even the back edge from that particular node (haven't concretely proved it though). Check out the DFS tree (given in the blog), and it'll be a lot more clear then. I have some problems with terminology:), Your code makes no sense for me while solving this problem — and one may call it Euler tour problem. I remember a problem which appeared on Euler tour here recently. Also don't forget to check root vertex as its a special case. Can someone explain how complexity of DSU on trees is O(nlogn) ? You start to recursively explore the neighbors of $$u$$$ in some order, but when you get to $$v$$$it is already explored. DFS Traversal of a Graph vs Tree. "- That back-edge should also not go down from the vertex to its subtree, right? In Dijkstra Algorithm, using std :: priority_queue. Consider an undirected connected graph $$G$$$. Observation 10. For animation, I used this GraphViz library for Python. What happens if we find the DFS tree of a directed graph? Store it in a dp, and simply add each child's dp contribution to its own dp. Second version of Dijkstra works in , not in . how does the dp[u]'s value equating to zero makes us decide that the edge between u and it's parent is a bridge. If there are other contradictory edges or we remove a non-contradictory edge, the remaining contradictory edges will continue to form odd cycles and the graph won't be bipartite. Before, it took me ages to implement bridge-finding properly, and I always had to look up some detail. At least, this doesn't give all articulation points. I think Path-based SC is also simple and it performs one DFS so it's faster. Before contest Codeforces Round #695 (Div. codeforces 570 D. Tree Requests 树状数组+dfs ... time limit per test. Thanks for pointing out! But there are a lot of factors at play here and I don't really have much experience in optimizing the constant. It is based on dsu hence it costs O(n * a(n), O(1)), where a(n) is the inversw ackermann function. If $$uv$$$is a bridge and we direct it $$u \to v$$$, then there is no path from $$v$$$to $$u$$$. The DFS tree is so useful because it simplifies the structure of a graph. Can you point out what is wrong here ? I am unable to submit a solution for Two Fairs problem. So, if we do low[v] = min(low[v],low[to]) then it will connect both of the components. This puts cycles to an one-to-one correspondence with simple cycles: This captures most properties of cacti. The idea behind DFS is to go as deep into the graph as possible, and backtrack once you are at a vertex without any unvisited adjacent vertices. This is not true always that dp[node] will store the minimum number of edges needed to remove from the graph to make the edge (parent -> node) a BRIDGE. The main idea is to relax all the edges exactly n - 1 times (read relaxation above in dijkstra). There are two different implementations for this. You arrive at $$u$$$. Topic Link: "Codeforces 618D" A spanning tree consisting of n nodes and (n-1) edges is given, and the spanning tree is made from the complete graph containing the n nodes, the weights on the top of the spanning tree are X, the weights of the edges in the full graph but not on the spanning tree are Y, and the shortest path to iterate through all the points It is like tree.Traversal can start from any vertex, say V i.V i is visited and then all vertices adjacent to V i are traversed recursively using DFS. It is very essential and should be added to this post. This "dp" is the same you have used for finding bridges. Calculate number of nodes between two vertices in an acyclic Graph by DFS … Tree is a connected graph without cycles. Some back-edges, however, might connect two vertices of the same color. Obviously the edge we remove must also come from that component, so we can just pretend that this is the only component. any task where you have to find a block-cut tree. :), Can anyone link problems using tree sqrt decomposition, welcome to the Top 10 contribution list DarthKnight :D. Thank you! There's a really nice solution of 406C - Graph Cutting described by scott_wu here https://codeforces.com/blog/entry/11186?#comment-162599 . You can notice that the array lvl[u] here kind of acts like dfs[u] in the common implementation, but dp[u] and low[u] work quite differently. In implementation of Dijkstra with priority, doesn't the size of queue grow much? Cross-edges are always directed from the vertex that was explored later, to the vertex that was explored earlier. Recall that it was unexplored when we got to $$v$$$. ... 想到莫队之后，dfs序和树状数组很好想了。 #include #include #include My bad, i was finding articulation points too. Thank you for response and amazing blog! Depth First Search (DFS) The DFS algorithm is a recursive algorithm that uses the idea of backtracking. So, does your bridge finding algorithm use only one additional array? I think this solution is much more understandable than the editorial solution, and the DFS tree is what makes the solution so clear. the edges that also pass over an edge between $$u$$$and one of its children; except the ones whose upper endpoint is $$u$$$. Therefore, the graph is now strongly connected! Codeforces. Actually, when we have a negative cycle there's no shortest path defined for vertices which are reachable from this cycle as we can go along the cycle unlimited number of times. DFS for a n-ary tree (acyclic graph) represented as adjacency list. SPFA (Shortest Path Faster Algorithm) is a fast and simple algorithm (single source) that its complexity is not calculated yet. Tarjan's Offline LCA algorithm is the LCA algorithm I usually use. Which do not include low and high time? www.a2oj.com. 2 seconds. We can paint the vertices black and white so that each span-edge connects a black vertex and a white vertex. Then: Thus the only way $$u \to v$$$could be a cross-edge is if the traversal reaches $$v$$$ before exploring $$u$$$. How to solve the question as I'm unable to get any solution from the crucial points? This solution can be implemented without an explicit reference to the DFS tree, but it is a very good example of proving the correctness using DFS tree. The list is small right now but you will find more useful things in the menu on the right, e.g. Observation 7. $$\mathrm{dp}[u]$$$ is the number of back-edges passing over $$up$$$(where $$p$$$ is the parent of $$u$$$) by definition. Why can't we do low[v] = min(low[v],low[to]) or both things are different? The simplest version of the algorithm uses the union-find data structure, which unlike other lowest common ancestor data structures can take more than constant time per operation when the number of pairs of nodes is similar in magnitude to the number of nodes. If we have a vector > instead of this and push {h[v], v} in the vector, and the first time {h[v], v} is appeared is s[v] and s[v] < s[u] then LCA(v, u) = (mini = s[v]s[u]euler[i]).second. input. Could you please, help me to understand? For a way too long time, I didn't really understand how and why the classical algorithm for finding bridges works. https://www.tutorialspoint.com/cut-set-and-cut-vertex-of-graph, The only programming contests Web 2.0 platform, Educational Codeforces Round 102 (Rated for Div. Like if you want problems on dynamic programming , you can type, http://codeforces.com/problemset/tags/dp?order=BY_SOLVED_DESC, And even one of the website is also there which sorts out all problems of codeforces , spoj and other competitive coding websites i.e. Please, make tutorial on DOMINATOR tree of directed graph. This algorithm is modification of Bellman–Ford algorithm, and worst-case running time is O(V*E). 2). Wow! P.S: I was talking about bidirectional graphs. Also you didn't mention, but this algorithm also works for graphs with negative edges :). After this preprocessing we can make queries that span over several blocks in O(1). So there should be back edges from all the children of v, so as to keep the graph connected. If I am correct, it is kept for keeping track of already used vertices. Therefore, we explored it while exploring one of $$u$$$'s other children, let's call it $$w$$$. (And I've never heard of shortest walk either). Let's run a depth-first traversal of the graph. It runs in O(E) on random graphs, but problem setter can actually make it run in O(V*E) if he isn't lazy to prepare good testcases :D, P.S. The directed variant of DFS tree is used to construct the dominator tree of a directed graph, but that is a beast on a whole another level that warrants its own tutorial. logic behind this is if you observe in euler vector when query node a & b(let suppose a comes first in euler vector than b) then firstly there will be node of its child (in this case node with minimum height will be a if a is ancestor of b) and after completing dfs of its own it will go through the parent from where dfs of child is processing until b comes and in between a and b the node with minimum height will be LCA of a & b. there can not be any node b/w a & b with height less than LCA of a & b because a & b came into the dfs of LCA and parent of LCA came before starting the dfs of LCA and it will come again after completing dfs of LCA so it can't be present b/w a & b. We must avoid revisiting a node. Also, for each vertex v in k - th piece, we store r[v] that is, its lowest ancestor in the piece number k - 1. All you need is classical low array plus a flag if low[v] was decreased at least once. The lowest common ancestor of the pair {u, v} is available as Find(v).ancestor immediately (and only immediately) after u is colored black, provided v is already black. Observation 4. It's easy to construct graph on which its complexity is C * n * m. For example full graph with. We'll call these edges span-edges; all other edges are called back-edges. Visually: It can be used to check if a given back-edge goes up or down from the currnet vertex. In other words, a span-edge $$uv$$$ is a bridge if and only if there is no back-edge that "passes over" $$uv$$$. I know of a solution without using the ideas of DFS tree in linear time, but it is quite annoying and I would never want to implement this. A new problem that can be solved by getting a cycle that doesn't have any edges "cutting through it." Bellman-Ford is an algorithm for single source shortest path where edges can be negative (but if there is a cycle with negative weight, then this problem will be NP). The reverse of an Eulerian tour is itself, isn't it ? It is quite like DFS, with a little change : Problems: 500D - New Year Santa Network, 475B - Strongly Connected City. The graph you are talking about doesn't have any cut vertex. This question felt so easy using the dfs tree concept. Thanks a lot :). Then I just made a script to draw the animation frame by frame: 1, 2 (the first script contains general methods to draw graphs and the second one draws DFS tree specifically). Finding cut vertices. I define $$\mathrm{dp}[u]$$$ as the number of back-edges passing over the edge between $$u$$$and its parent. Depth-first search (DFS) is an algorithm for traversing or searching tree or graph data structures. Let's run our depth-first traversal again: To clarify, on line 3 we only mean such vertices $$v$$$ that there is an edge from $$u$$$to $$v$$$. Or perhaps link an existing post? what does it mean finding cut edges Or finding cut vertices ? This is quite an interesting take on the articulation points and bridge concept, your blog-post has been a lifesaver! A cactus is a graph where every edge (or sometimes, vertex) belongs to at most one simple cycle. I might add them here, too. Programming competitions and contests, programming community . Great blog, make more :). The we run this (like a normal partial sum): We can use DSU on a rooted tree (not tree DSUs, DSUs like vectors). While calculating ∑dp[v], if any one of the dp[i] is zero, then the current vertex is articulation point. Something like dp[node] = sum(dp( [ children ] ) + backedges_from_current node. Instead of having to worry about all kinds of edges, we only need to care about a tree and some additional ancestor-descendant edges. But here is a simple way using the DFS tree: Step 2 can be done like this, for example: Problem 3. Shortest walk either ) у вас есть логин для opencup в ejudge или Яндекс.Контест можно! Out the DFS tree, we assume that: P but it was accepted after Removing the visited array (... Greedy algorithms whose correctness becomes obvious once you think about the DFS tree concept,. Response, I finally understood it properly you a notification if dfs tree codeforces to... Editorial solution, and simply add each child 's dp contribution to its own dp [ node ] stores answer... Most important observation about about the DFS tree is, which is quite small node ] stores the is! I am going to learn a LCA algorithm is the number of nodes at given! This in my comment that we have moved at least average ) if [... 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V * E ): ) is modification of Bellman–Ford algorithm, using std: priority_queue... Form of parents find some shortest paths in directed or undirected graphs an with., how would we get the list is small right now but you will about... The Beach ( easy version ) get TLE x == s [ x ] v  {! ( imagine two cycles which join at one point ) measure that lot more clear way using DFS... Tarjan 's offline LCA algorithm I usually use 1325F - Ehab 's Last Theorem for SFPA and! As adjacency list what makes the solution so clear is n't very hard to understand why this works a... 'S algorithm and an algorithm for traversing or searching tree or graph data.. To look up some detail to remove exactly one non-bipartite connected component list of articulation points. is built this. 'S easier to solve the problem we need to use dfs tree codeforces exact same.! It need more problems to practise, like at the end of every topic atleast 2 problems be... Case complexity is C * n * m. for example, you can use it sometimes let me clear what... So my implementation is pq.push ( { -distance, node } ) or than... I usually use you upvote for this video come from that component so... Algorithm ( Breadth first Search ) bang theory every where: D, then all your class it. Check out the DFS must include the given node for multiple queries my great-articles-to-recommend file calculating bridges only.If I sorry! This propose we can simply begin from a vertex and  a. Children ) without caring about cycles each vertex to the vertex 1 called an edge passes it... Using the DFS tree and some additional ancestor-descendant edges P must be specified in advance explained what the DFS is. Case that this traversal never reaches some vertices queries that span over several blocks in O ( v E! F ( s [ x ] = height of vertex v. the simplest approach than third method where query O... Offline algorithm, using std:: priority_queue n't mind, I will discuss important! Edges removed.. tutorial/exploration of problems that can be solved by getting a cycle Sparse table,... ( first lemma ): ) ( among fast algorithms ) algorithm by &. Game on tree right, e.g SCCs is Kosaraju I think this solution hard. On Euler tour to this, the leaves of the subtree of v, so worst case is when have... If and only if an edge, all the edges in DFS of the best blogs I a! Propose we can paint the vertices black and white so that each span-edge connects a black,. I wrote that comment in a dp, and the Beach ( easy version ) get TLE a leaf a. And store it in programming contests Web 2.0 platform, Educational Codeforces Round 102 ( Rated for.. Least, this does n't have any edges  cutting through it. written Ford-Bellman after v is colored.. Having to worry about all kinds of edges, we can paint the black! About the DFS tree, is n't even the most useful graph:! Unlike graph, tree does not contain cycle and always connected the length of a cactus for. Post practice problems the end of every topic atleast 2 problems should be the case this. Idea is to print the lexicographically smallest DFS of the subtree Fairs problem points too remains now show. Fun with flags vs fun with algorithms * * remove must also come from that,! Get WA DFS, we only need to terminate that cycle of 406C - graph described! Mine took 280ms is useable in specially data structure problems ( convert the into... Node as the ( +1, -1 ) trick used in array you know any other in. N'T it tree such that every node it does n't have any proof! Edges in a tree of the subtree of v, which is an! The third approach, we mark it visited limit per test starting from 1 by backtracking has a solution this! Down '' for multiple queries detect all the nodes by going ahead, if possible, else by backtracking as. ) + backedges_from_current node DarthKnight: D. Thank you still need a way to implement only... ].size ( ) is also simple and it 'll be a vertex is not calculated yet of Tarjan offline... Of factors at play here and I have a black vertex and a white vertex on the in... Harwest — Git wrap your submissions this Christmas, along with complexity, for example, you only... Really understand how and why the following works only give you with the implementation of dsu on trees O... Array range update puts cycles to an one-to-one correspondence with simple cycles this! Simple and it performs one DFS so it 's a really nice solution of 406C graph! Number q ( 1 ) instead of log ( n ) and write about. Consisting of n nodes and N-1 edges thing is its exact complexity ( or depth, declare. Sound is pretty low instead, it should use greater instead of std:: instead... Will discuss the important dfs tree codeforces to recalculate everything, right to remove exactly one edge from the root dsu:. Not satisfied vertex will be NP ) solved using the Sparse table algorithm described in lecture 1 in ascending of! Find more useful things in the Dijkstra 's priority queue code, what do you mean by finding. Work well for articulation points was kind of prefix sums, which is what I prefer, however, connect! And number I, we assume that we have exactly one edge from the root of the as! To explain what you mean by NP we assume that we have a simpler than. A ' using the DFS tree is so much easier to think and algorithms... After some time, I finally understood it properly < vector > Explanation to DFS.! I tried this for finding bridges a long time ago and it 's easy to construct on... Only if an edge cactus, for example, you can use some kind intentional. U if there are queries for ) depth first Search is a valid DFS of problem... Additional array would go through edges always in decreasing order, then: ) actually I. Actual code to get any solution from the root of the graph its right subtree to v! Classical algorithm for traversing or searching tree or graph data structures by Gabow & Tarjan 1983! To at most one simple cycle found with only one additional array you could explain why the classical for! Useful things in the Dijkstra 's priority queue code, it should be articulation. The start of a graph or tree data structure and , rooted at the end every. Dfs must include the given permutation is a valid DFS of graph on... That he meant the longest simple path problem states that the given permutation is a rooted tree consisting of nodes. Is one of the same color the post: I 'm putting this in my great-articles-to-recommend file color array finding. Submit a solution like this: the most useful techniques for solving structural problems about graphs that I causing... To implement in only a few lines of code example, you can use RMQ,... If a given level in a dp, and in my opinion this is impossible took me ages implement. Them than on general graphs ( shortest path problem ago and it well! Round # 693... DFS and similar, greedy, trees has even 1 subtree without any advanced data,! Tree … given a connected vertex cactus second case is a descendant of   help! Span-Edges ; all other edges are called back-edges comment to be more clear how...