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Topological Sort

Ugliest Unicorn answered on July 25, 2022 Popularity 10/10 Helpfulness 1/10

answer Topological Sort

related What is topological sort?

Topological Sort

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import java.util.Map;
import java.util.Queue;
import java.util.List;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.ArrayDeque;

private static String[] topologicalSort(Map<String, String[]> digraph) {
    // digraph is a map:
    //   key: a node
    // value: an array of adjacent neighboring nodes

    // construct a hash mapping nodes to their indegrees
    Map<String, Integer> indegrees = new HashMap<>();
    for (String node : digraph.keySet()) {
        indegrees.put(node, 0);
    }
    for (Map.Entry<String, String[]> entry : digraph.entrySet()) {
        String node = entry.getKey();
        for (String neighbor : entry.getValue()) {
            indegrees.put(neighbor, indegrees.get(neighbor) + 1);
        }
    }

    // track nodes with no incoming edges
    Queue<String> nodesWithNoIncomingEdges = new ArrayDeque<>();
    for(Map.Entry<String, Integer> entry : indegrees.entrySet()) {
        if (entry.getValue() == 0) {
            nodesWithNoIncomingEdges.add(entry.getKey());
        }
    }

    // initially, no nodes in our ordering
    List<String> topologicalOrdering = new ArrayList<>();
    
    // as long as there are nodes with no incoming edges that can be
    // added to the ordering
    while (nodesWithNoIncomingEdges.size() > 0) {

        // add one of those nodes to the ordering
        String node = nodesWithNoIncomingEdges.poll();
        topologicalOrdering.add(node);

        // decremenet the indegree of that node's neighbors
        for (String neighbor : digraph.get(node)) {
            indegrees.put(neighbor, indegrees.get(neighbor) - 1);
            if (indegrees.get(neighbor) == 0) {
                nodesWithNoIncomingEdges.add(neighbor);
            }
        }
    }

    // we've run out of nodes with no incoming edges
    // did we add all the nodes or find a cycle? 
    if (topologicalOrdering.size() != digraph.size()) {
        throw new IllegalArgumentException("Graph has a cycle! No topological ordering exists.");
    }

    return topologicalOrdering.toArray(new String[topologicalOrdering.size()]);
}

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Tags: java topological-sort

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Contributed on Sep 26 2023

Ugliest Unicorn

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What is topological sort?

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Topological sort: 
outlines all the paths from point A to point B in a directed acyclic graph. 
Answers the question can I go from point A to point B and if yes, how?
C-> A ->B ->D
you CAN go from C to A
you CAN go from A to D
you CANNOT go from D to C

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Contributed on Jul 25 2022

Javasper

0 Answers Avg Quality 2/10

Topological Sort

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