Skip to content

Instantly share code, notes, and snippets.

@raymondchua
Last active December 15, 2019 09:50
Show Gist options
  • Save raymondchua/7954072 to your computer and use it in GitHub Desktop.
Save raymondchua/7954072 to your computer and use it in GitHub Desktop.
Java's implementation of Dijkstra's Algorithm
import java.util.PriorityQueue;
import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
public class DijkstraAlgo{
/* Dijkstra Algorithm
*
*
*/
public static void computePaths(Node source){
source.shortestDistance=0;
//implement a priority queue
PriorityQueue<Node> queue = new PriorityQueue<Node>();
queue.add(source);
while(!queue.isEmpty()){
Node u = queue.poll();
/*visit the adjacencies, starting from
the nearest node(smallest shortestDistance)*/
for(Edge e: u.adjacencies){
Node v = e.target;
double weight = e.weight;
//relax(u,v,weight)
double distanceFromU = u.shortestDistance+weight;
if(distanceFromU<v.shortestDistance){
/*remove v from queue for updating
the shortestDistance value*/
queue.remove(v);
v.shortestDistance = distanceFromU;
v.parent = u;
queue.add(v);
}
}
}
}
public static List<Node> getShortestPathTo(Node target){
//trace path from target to source
List<Node> path = new ArrayList<Node>();
for(Node node = target; node!=null; node = node.parent){
path.add(node);
}
//reverse the order such that it will be from source to target
Collections.reverse(path);
return path;
}
public static void main(String[] args){
//initialize the graph base on the Romania map
Node n1 = new Node("Arad");
Node n2 = new Node("Zerind");
Node n3 = new Node("Oradea");
Node n4 = new Node("Sibiu");
Node n5 = new Node("Fagaras");
Node n6 = new Node("Rimnicu Vilcea");
Node n7 = new Node("Pitesti");
Node n8 = new Node("Timisoara");
Node n9 = new Node("Lugoj");
Node n10 = new Node("Mehadia");
Node n11 = new Node("Drobeta");
Node n12 = new Node("Craiova");
Node n13 = new Node("Bucharest");
Node n14 = new Node("Giurgiu");
//initialize the edges
n1.adjacencies = new Edge[]{
new Edge(n2,75),
new Edge(n4,140),
new Edge(n8,118)
};
n2.adjacencies = new Edge[]{
new Edge(n1,75),
new Edge(n3,71)
};
n3.adjacencies = new Edge[]{
new Edge(n2,71),
new Edge(n4,151)
};
n4.adjacencies = new Edge[]{
new Edge(n1,140),
new Edge(n5,99),
new Edge(n3,151),
new Edge(n6,80),
};
n5.adjacencies = new Edge[]{
new Edge(n4,99),
new Edge(n13,211)
};
n6.adjacencies = new Edge[]{
new Edge(n4,80),
new Edge(n7,97),
new Edge(n12,146)
};
n7.adjacencies = new Edge[]{
new Edge(n6,97),
new Edge(n13,101),
new Edge(n12,138)
};
n8.adjacencies = new Edge[]{
new Edge(n1,118),
new Edge(n9,111)
};
n9.adjacencies = new Edge[]{
new Edge(n8,111),
new Edge(n10,70)
};
n10.adjacencies = new Edge[]{
new Edge(n9,70),
new Edge(n11,75)
};
n11.adjacencies = new Edge[]{
new Edge(n10,75),
new Edge(n12,120)
};
n12.adjacencies = new Edge[]{
new Edge(n11,120),
new Edge(n6,146),
new Edge(n7,138)
};
n13.adjacencies = new Edge[]{
new Edge(n7,101),
new Edge(n14,90),
new Edge(n5,211)
};
n14.adjacencies = new Edge[]{
new Edge(n13,90)
};
Node[] nodes = {n1,n2,n3,n4,n5,n6,n7,n8,n9,n10,n11,n12,n13,n14};
//compute paths
computePaths(n1);
//print shortest paths
/*
for(Node n: nodes){
System.out.println("Distance to " +
n + ": " + n.shortestDistance);
List<Node> path = getShortestPathTo(n);
System.out.println("Path: " + path);
}*/
List<Node> path = getShortestPathTo(n13);
System.out.println("Path: " + path);
}
}
//define Node
class Node implements Comparable<Node>{
public final String value;
public Edge[] adjacencies;
public double shortestDistance = Double.POSITIVE_INFINITY;
public Node parent;
public Node(String val){
value = val;
}
public String toString(){
return value;
}
public int compareTo(Node other){
return Double.compare(shortestDistance, other.shortestDistance);
}
}
//define Edge
class Edge{
public final Node target;
public final double weight;
public Edge(Node targetNode, double weightVal){
target = targetNode;
weight = weightVal;
}
}
@meesont
Copy link

meesont commented May 8, 2019

Why do you make an array of nodes on line 159? This isn't called or used and serves no function

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment