What is the easiest way to implement prim's algorithm in java?

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1

I tried to implement prim’s algorithm in java but I found it difficult to implement. Can any one help me to implement prim’s algorithm using built in data structure(Collections framework)

2

For starters you can have a look at the following links:-

Prims code in java

Java prims implementation

you can have a look at both the links but, i think this would not work for extremely large data sets as both of the codes maintain a adjacency matrix for the graph, for large data sets you have to maintain either a 2-D pair or a vector of pairs

3

Here’s the code
import java.util.*;

public class PRIMS {

static class EDGE{
    int from, to;
    double weight;
    
    EDGE(int f, int t, double w){
        from = f;
        to = t;
        weight = w;
    }
}

public static ArrayList<EDGE> prims(ArrayList<ArrayList<EDGE>> G){
    if(G.isEmpty())throw new NullPointerException("The Graph is empty");
    
    ArrayList<EDGE> mst = new ArrayList<>();
    PriorityQueue<EDGE> pq = new PriorityQueue<>((Object o1, Object o2) -> {
        EDGE first = (EDGE)o1;
        EDGE second = (EDGE)o2;
        
        if(first.weight<second.weight)return -1;
        else if(first.weight>second.weight)return 1;
        else return 0;
    });
    
    for(EDGE e:G.get(0)){
        pq.add(e);
    } 
    
    boolean[] marked = new boolean[G.size()];
    marked[0] = true;
    while(!pq.isEmpty()){
        EDGE e = pq.peek();
        
        pq.poll();
        if(marked[e.from] && marked[e.to])continue;
        marked[e.from] = true;  
        for(EDGE edge:G.get(e.to)){
            if(!marked[edge.to]){
                pq.add(edge);  
            }
        }
        marked[e.to] = true; 
        mst.add(e);
        
    }
    return mst;
}

public static ArrayList<ArrayList<EDGE>> createGraph(EDGE[] edges){
    ArrayList<ArrayList<EDGE>> G = new ArrayList<>();
    int length = edges.length*2;
    for(int i=0;i<length;++i){
        G.add(new ArrayList<>());
    }
    
    for(EDGE e:edges){
        EDGE other = new EDGE(e.to, e.from, e.weight);
        G.get(e.from).add(e);
        G.get(e.to).add(other);
        System.out.println("Added edge ["+e.from+", "+e.to+" : "+e.weight+"] "+"["+e.to+", "+e.from+" : "+e.weight+"]");
    }
    
    return G; 
}

public static void main(String[] args){
    EDGE[] edges = new EDGE[16];
    
    edges[0] = new EDGE(0, 7, 0.16);
    edges[1] = new EDGE(2, 3, 0.17);
    edges[2] = new EDGE(1, 7, 0.19);
    edges[3] = new EDGE(0, 2, 0.26);
    
    edges[4] = new EDGE(5, 7, 0.28);
    edges[5] = new EDGE(1, 3, 0.29);
    edges[6] = new EDGE(1, 5, 0.32);
    edges[7] = new EDGE(2, 7, 0.34);
    
    edges[8] = new EDGE(4, 5, 0.35);
    edges[9] = new EDGE(1, 2, 0.36);
    edges[10] = new EDGE(4, 7, 0.37);
    edges[11] = new EDGE(0, 4, 0.38);
    
    edges[12] = new EDGE(6, 2, 0.40);
    edges[13] = new EDGE(3, 6, 0.52);
    edges[14] = new EDGE(6, 0, 0.58);
    edges[15] = new EDGE(6, 4, 0.93);
    
    ArrayList<ArrayList<EDGE>> graph = createGraph(edges);
    ArrayList<EDGE> mst = prims(graph);
    
    System.out.println("MST: ");
    for(EDGE e:mst){
        System.out.println(e.from+" - "+e.to+" : "+e.weight);
    } 
}

}

1 Like
4

Thanks a lottt !! :star_struck: