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Questions tagged [graph-traversal]

Questions about graph traversal algorithms such as BFS and DFS.

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174 views

Can the pre-order traversal of two different trees be the same even though they are different?

This question pretty much explains that they can, but does not show any examples of there being two different trees with the same pre-order traversal. It is also mentioned that the in-order ...
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1answer
23 views

Minimize sequence storage by overlapping prefixes

I bumped into this problem today, and after a bit of pondering, I think I have a solution in $O(n^3)$, which is better than no solution or an $O(n!)$ solution, but my answer still isn't great. Can ...
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1answer
46 views

Cannibals missionaries problem - solving usings graphs

I am trying to solve the cannibals - missionaries problem; we have the number of cannibals, the number of missionaries and the position of the boat. We are trying to transfer all of them to the other ...
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2answers
61 views

Minimize number of DFS searches in a graph

I got a weird homework question about graph. A helicopter is going to land on an island to check the n houses after an earthquake. Some of the two-way roads connecting the houses are destroyed ...
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67 views

Maximum size of BFS open set on a grid

I have a 2D grid of infinite size that can either be 4-connected or 8-connected (as defined in https://en.wikipedia.org/wiki/Pixel_connectivity). I am implementing breadth-first search on this grid ...
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1answer
86 views

Subtree with minimum sum of nodes' costs

Let's consider a tree with root $r$ ( not necessary binary) and to each node $i$ we associate a cost $\sigma(i)$ that can be negative, positive or zero. We want to select the set of nodes that ...
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1answer
47 views

Define the time complexity of Kruskal's algorithm as function

I am trying to define the time complexity of Kruskal's algorithm as function dependant on: the number of vertices V the number of edges ...
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26 views

Are these algorithms for detecting cycles in directional graph correct?

I want to detect whether a subset of a directional graph reachable from a given root has a cycle, and print some useful debug information about the cycle. It's not a problem if there's a cycle not ...
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1answer
31 views

Is a subgraph of G always connected

I am trying to figure out if given a connect graph with N nodes and A edges, its subgraphs are connected. In order word: given a graph G, can I have a subgraph of G that is not connected? Or: can a ...
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1answer
37 views

Idea for a Graph Based Algorithm

Assume there is a Shopping Mall x and all the roads in the city are one way such that, irrespective of the path you take starting from x, you will always end up at vertex x. Device an algorithm to ...
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2answers
70 views

Fewest traversals to visit all vertices of DAG

I want to find the fewest traversals to visit all vertices of a DAG. To take a very simple case: ...
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3answers
1k views

What is the meaning of 'breadth' in breadth first search?

I was learning about breadth first search and a question came in my mind that why BFS is called so. In the book Introduction to Algorithms by CLRS, I read the following reason for this: Breadth-...
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1answer
47 views

Directed graph where DFS returns on a node before all its child nodes are visited?

Give an example of a directed graph in which a depth-first search backs up from a vertex $v$ before all the vertices that can be reached from $v$ via one or more edges are discovered. My professor ...
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15 views

Implementation of multiple sink shortest pair of disjoint paths problem for multigraphs

I would like to implement the shortest pairs of edge-disjoint paths of Suurballe and Tarjan for multigraphs in the interpretation of Banerjee et al. (http://web.cs.iastate.edu/~pavan/papers/short.pdf, ...
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1answer
39 views

A problem to maximize the number of edges in a cycle while minimizing the total weight

I encountered the problem below and the only solution I came up with is branch and bound like that is used in TSP and I don’t think the bound I used is good enough. Are there any better idea on this? ...
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11 views

Find a path passing through the minimum number of odd nodes [duplicate]

Assume you have a undirected graph where each vertex is marked either odd or even. Given two even nodes, How do you find the path that passes through the minimum number of odd nodes in O(edges)? Now ...
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61 views

Algorithm for finding single input/output sub graphs

I'm running into an interesting directed acyclic graph (DAG) problem and was wondering if this is a known problem and if it has an efficient algorithm for it. I will use 'graph' and 'DAG' ...
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1answer
17 views

Term for an A*-like pathfinding strategy where only the heuristic goal distance matters

I am trying to find a proper term for the A*-like best-first pathfinding strategy where the node to expand next is the one with the least estimated distance from the goal, regardless of its distance ...
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1answer
90 views

Finding topologically sorted connected components in directed acyclic graph

I am aware topological sort and connected component algorithms are very related, but I have been looking for an algorithm to simultaneously compute both, rather than one after the other, and I am ...
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1answer
36 views

Devising a way to spot a contradiction given a set of statements using graphs

If we had statements like: John is as tall as Mark, Mark is as tall as Sally, Chuck is as tall as Sally, Chuck is shorter than John. Would there be a way to figure out that there is a contradiction ...
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2answers
123 views

Solving a in/equality constraint problem with graph search

You are given a list of m constraints over n distinct variables x1, ..., xn. Each constraint is of one of the following two types. An equality constraint of the form xi = xj for some i!=j. An ...
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2answers
82 views

Find maximal subgraph containing only nodes of degree 2 and 3

I'm trying to implement a (Unweighted) Feedback Vertex Set approximation algorithm from the following paper: FVS-Approximation-Paper. One of the steps of the algorithm (described on page 4) is to ...
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34 views

$n$ machines, $n$ types of jobs with $q_j$ jobs, minimizing the cost

I have this problem A firm has $q_j$ jobs of type $j$, where $1 \leq j \leq n$. It also has $[n] = {1,2,...n}$ machines. Machine $i$ can service any job of type $j$ where $j ≤ i$. The cost of ...
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294 views

Steps that guarantee exiting a maze

Given a 2-dimensional maze where you can give 4 commands "move up/down/right/left". Knowing the maze but not where the person is, how to find the minimum sequence of commands that guarantees exiting ...
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1answer
32 views

Shortest path in graph by flipping binary colored nodes to one color [closed]

Given a graph consists of two-colored nodes(e.g. white and black) and a starting node, and every time you visit a node, its color is switched(from black to white, or, white to black), how to find the ...
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1answer
45 views

How to perform local search to find maximal induced subgraphs?

I'm looking for efficient ways to perform local search to find maximal induced subgraphs that satisfy certain properties : a tree, a forest or a bipartite subgraph for example. What I mean by local ...
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28 views

Order of Traversal in Bidirectional Dataflow Analysis

Does order of traversing CFG matter in solving bidirectional dataflow problems, from a performance perspective?
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58 views

Quick and space-efficient way to find whether two sets intersect

I hope you can help me - Given a lot of sets containing integers, I'd like for any two sets, to quickly (i.e. O(1)) ask whether they intersect. Note that I don'...
3
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2answers
62 views

When searching graphs breadth-first and depth-first color the graph. Who un-colors the graph when the search is completed?

The depth-first and breadth-first algorithms for traversing graphs use a flag (or color) to mark nodes when they are visited. For example some forms of the algorithms use white/grey/black as colors. ...
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2answers
72 views

The complexity of the reachability-coreachability analysis of a finite state machine

Let $A = \{\Sigma,Q,\delta,q_{0}, Q_{m}\}$ be a finite state machine (FSM). A state $q \in Q$ is reachable if there exists a string $s \in \Sigma^{*}$ such that $\delta(q_{0},s) = q$. The state $q$ is ...
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2answers
78 views

“Process functions” in iterative and recursive depth-first search

I was reading The Algorithm Design Manual by Steven Skiena, and I noticed his use of "process functions" in depth-first search and breadth-first search. Consider the following pseudocode for depth-...
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1answer
61 views

Least number of guesses needed to determine all unknown subsets of a set

Say I have a set $\mathbb{S}=\{1,2,...,n\}$. I have an adversary who breaks up $\mathbb{S}$ into $k$ unknown and disjoint subsets. Denote this new set $\mathbb{A}$. I can guess any combination $s$ and ...
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1answer
154 views

Shortest path with a start vertex that touches all nodes at least once with repeats allowed

I tried looking this problem up for quite a bit now, but can't seem to find a whole lot of discussion about this. At first it sounded like the TSP to me, but I don't think so (it's much harder to do I ...
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2answers
80 views

Reduce the total internal border of a set of touching rectangles (using graphs)

I have a set of touching rectangles (Initial problem), and an associated graph relating the rectangles through the edges. I want to reduce the rectangles through graph operations to the minimum ...
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2answers
210 views

Minimum Distance Spanning Tree Dijkstra

I would like to construct a Minimum Distance Spanning Tree (Dijkstra) for the graph below: MDST: {(a,c), (c,h), (c,f), (a,d), (h,g), (a,b), (d,e), (h,j), (h,i), (j,k), (e,m), (i,l)} Is my ...
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0answers
58 views

Enumerate all re-convergent paths in a digraph (with cycles)

I'm looking for an algorithm to count and enumerate (separately, if differing complexity) all the reconvergent pathsets in a simple, directed, non-weighted graph, which may contain cycles. That is, ...
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1answer
444 views

DFS & BFS Spanning Trees

I want to construct a DFS and a BFS spanning trees for the graph below. The root is node a. At each step the next edge to be traversed should be the cheapest one. DFS: My understanding that to the ...
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2answers
187 views

How to traverse a graph in reverse with dfs

So I'm watching Stanford's algorithm lectures and I'm on Kosaraju's algorithm. In the lecture, the algorthm was given in 3 steps: calculate the graph with all arcs reversed, run dfs on reversed graph, ...
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2answers
124 views

Proof for BFS and DFS equivalence

I'm trying to prove (by induction) that BFS in equivalent to DFS, in the sense that they return the same set of visited nodes, but I'm stuck in the middle of some of the cases. Let $G$ be a directed ...
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1answer
44 views

proof that BFS remains total after adding edge to graph

I'm trying to prove that if $G$ is a connected graph, then $BFS(u\in G)$ is total (i.e. it visits all the vertices of $G$). The inductive proof consists in 2 cases: (i) Prove that $\rm{BFS}$$(u \in \...
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1answer
101 views

Post numbers in DFS tree of an undirected graph

How could you prove: An edge (u,v) is part of an undirected graph G. If post(u) $<$ post(v) (i.e. the post number of u is smaller than that of v) then it implies that v is an ancestor of u in the ...
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2answers
177 views

Is there a name for this graph search strategy?

The standard implementation of breadth-first search looks like ...
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2answers
58 views

Calculation of Inorder Traversal Complexity

I want to analyze complexity of traversing a BST. I directly thought that its complexity as $O(2^n)$ because there are two recursive cases. I mean $T(n) = constants + 2T(n-1)$. However, AFAI research ...
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1answer
121 views

Understanding the “ordering of the four types of edges” in DFS

The following is Exercise 22.3-6 from CLRS (Introduction to Algorithms, the 3rd edition; Page 611). Show that in an undirected graph, classifying an edge $(u,v)$ as a tree edge or a back edge ...
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14 views

Alternative technologies/theories for a “Git for Structured Graph Databases”

Along the lines of detect differences between tree structures, I'm wondering if there are any general techniques or tools (e.g. the techniques behind the tools) that can essentially do what Git does ...
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1answer
69 views

Graph Traversal Solutions for “Find all unique paths” Problem

I was studying the grid problem where a robot is at the top left position and wants to go to the bottom right position and you need to return the number of unique paths it can take to get there with ...
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236 views

A longest path can be found using DFS

The following is a Yes/No question. In an undirected unweighted graph $G$, a longest path between two given vertices $s$ and $t$ can be found using Depth-first search? If it is possible to use DFS ...
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1answer
113 views

DFS on reversed graph

I was watching a lecture on determining the strongly connected components of a graph using Kosaraju's algorithm and the lecturer claimed one can easily walk the edges in reverse fashion. While I see ...
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2answers
1k views

Proving Postorder Traversal's Time Complexity

I am looking at the following algorithm for performing a Postorder Traversal of a binary tree ...