Added the implementation of the so far missing Bellman-Ford algorithm.
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PalAditya
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5921efc83e
158
DataStructures/Graphs/BellmanFord.java
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158
DataStructures/Graphs/BellmanFord.java
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import java.util.*;
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class BellmanFord
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/*Implementation of Bellman ford to detect negative cycles. Graph accepts inputs in form of edges which have
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start vertex, end vertes and weights. Vertices should be labelled with a number between 0 and total number of vertices-1,both inclusive*/
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{
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int vertex,edge;
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private Edge edges[];
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private int index=0;
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BellmanFord(int v,int e)
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{
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vertex=v;
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edge=e;
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edges=new Edge[e];
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}
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class Edge
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{
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int u,v;
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int w;
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/**
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*@param u Source Vertex
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* @param v End vertex
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* @param c Weight
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*/
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Edge(int a,int b,int c)
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{
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u=a;
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v=b;
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w=c;
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}
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}
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/**
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* @param p[] Parent array which shows updates in edges
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* @param i Current vertex under consideration
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*/
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void printPath(int p[],int i)
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{
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if(p[i]==-1)//Found the path back to parent
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return;
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printPath(p,p[i]);
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System.out.print(i+" ");
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}
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public static void main(String args[])
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{
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BellmanFord obj=new BellmanFord(0,0);//Dummy object to call nonstatic variables
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obj.go();
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}
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public void go()//Interactive run for understanding the class first time. Assumes source vertex is 0 and shows distaance to all vertices
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{
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Scanner sc=new Scanner(System.in);//Grab scanner object for user input
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int i,v,e,u,ve,w,j,neg=0;
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System.out.println("Enter no. of vertices and edges please");
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v=sc.nextInt();
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e=sc.nextInt();
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Edge arr[]=new Edge[e];//Array of edges
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System.out.println("Input edges");
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for(i=0;i<e;i++)
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{
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u=sc.nextInt();
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ve=sc.nextInt();
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w=sc.nextInt();
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arr[i]=new Edge(u,ve,w);
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}
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int dist[]=new int[v];//Distance array for holding the finalized shortest path distance between source and all vertices
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int p[]=new int[v];//Parent array for holding the paths
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for(i=0;i<v;i++)
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dist[i]=Integer.MAX_VALUE;//Initializing distance values
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dist[0]=0;
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p[0]=-1;
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for(i=0;i<v-1;i++)
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{
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for(j=0;j<e;j++)
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{
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if((int)dist[arr[j].u]!=Integer.MAX_VALUE&&dist[arr[j].v]>dist[arr[j].u]+arr[j].w)
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{
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dist[arr[j].v]=dist[arr[j].u]+arr[j].w;//Update
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p[arr[j].v]=arr[j].u;
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}
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}
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}
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//Final cycle for negative checking
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for(j=0;j<e;j++)
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if((int)dist[arr[j].u]!=Integer.MAX_VALUE&&dist[arr[j].v]>dist[arr[j].u]+arr[j].w)
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{
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neg=1;
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System.out.println("Negative cycle");
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break;
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}
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if(neg==0)//Go ahead and show results of computaion
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{
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System.out.println("Distances are: ");
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for(i=0;i<v;i++)
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System.out.println(i+" "+dist[i]);
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System.out.println("Path followed:");
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for(i=0;i<v;i++)
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{
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System.out.print("0 ");
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printPath(p,i);
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System.out.println();
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}
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}
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}
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/**
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* @param source Starting vertex
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* @param end Ending vertex
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* @param Edge Array of edges
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*/
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public void show(int source,int end, Edge arr[])//Just shows results of computation, if graph is passed to it. The graph should
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//be created by using addEdge() method and passed by calling getEdgeArray() method
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{
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int i,j,v=vertex,e=edge,neg=0;
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double dist[]=new double[v];//Distance array for holding the finalized shortest path distance between source and all vertices
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int p[]=new int[v];//Parent array for holding the paths
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for(i=0;i<v;i++)
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dist[i]=Integer.MAX_VALUE;//Initializing distance values
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dist[source]=0;
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p[source]=-1;
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for(i=0;i<v-1;i++)
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{
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for(j=0;j<e;j++)
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{
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if((int)dist[arr[j].u]!=Integer.MAX_VALUE&&dist[arr[j].v]>dist[arr[j].u]+arr[j].w)
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{
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dist[arr[j].v]=dist[arr[j].u]+arr[j].w;//Update
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p[arr[j].v]=arr[j].u;
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}
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}
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}
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//Final cycle for negative checking
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for(j=0;j<e;j++)
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if((int)dist[arr[j].u]!=Integer.MAX_VALUE&&dist[arr[j].v]>dist[arr[j].u]+arr[j].w)
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{
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neg=1;
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System.out.println("Negative cycle");
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break;
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}
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if(neg==0)//Go ahead and show results of computaion
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{
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System.out.println("Distance is: "+dist[end]);
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System.out.println("Path followed:");
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System.out.print(source+" ");
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printPath(p,end);
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System.out.println();
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}
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}
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/**
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*@param x Source Vertex
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* @param y End vertex
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* @param z Weight
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*/
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public void addEdge(int x,int y,int z)//Adds unidirectionl Edge
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{
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edges[index++]=new Edge(x,y,z);
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}
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public Edge[] getEdgeArray()
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{
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return edges;
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}
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}
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