Wrong answer while calculating Minimum Spanning Tree Using Kruskal Algorithm

Question

I am Trying to solve this MST question on spoj using kruskal algorithm. my program seems to work on all test cases but spoj repeatedly is giving WA on this code.

I am not able to find any failing test cases on this code. Can someone please point out to what I am doing wrong.

import java.io.PrintWriter;
import java.util.Arrays;


public class CSTREET {

    static final int MAX = 1002;
    static Node edgeList[];
    static int parent[] = new int[MAX];


    public static void main(String[] args) throws Exception {
        Reader in = new Reader();
        PrintWriter out = new PrintWriter(System.out, true);
        int t = in.nextInt();
        while (t-- != 0) {

            int price = in.nextInt();
            int vertices = in.nextInt();
            int edge = in.nextInt();
            int idx = 0;
            edgeList = new Node[edge];
            for (int i = 1; i <= vertices; i++) {
                parent[i] = i;
            }

            while (idx < edge) {

                int src = in.nextInt();
                int dest = in.nextInt();
                int cost = in.nextInt();
                Node node = new Node(src, dest, cost);

                edgeList[idx] = node;
                idx++;
            }

            Arrays.sort(edgeList);
            int edgeCount = 0;


            long totalCost = 0;
            idx = 0;

            while (edgeCount < vertices-1 ) {
                Node curEdge = edgeList[idx];
                if (!checkCycle(curEdge.src, curEdge.dest)) {

                    edgeCount++;
                    totalCost += curEdge.cost;

                }
                idx++;

            }
            out.println(totalCost * price);
        }
    }


    static boolean checkCycle(int src, int dest) {

        if (findParent(src) == findParent(dest)) {
            return true;
        }

        while (parent[dest] != parent[src]) {
            parent[dest] = src;
            src = parent[src];
        }

        return false;

    }

    static int findParent(int i) {

        while (parent[i] != i) {
            i = parent[i];
        }

        return i;
    }


    static class Node implements Comparable<Node> {

        int src;
        int dest;
        int cost;

        public Node(int src, int dest, int cost) {
            this.src = src;
            this.dest = dest;
            this.cost = cost;
        }

        @Override
        public int compareTo(Node o) {
            return this.cost - o.cost;
        }
    }
}

Please put the code that you are actually submitting. I get compile error when I submit this code. — Saeid
– Saeid, Commented Oct 1, 2016 at 18:17

Saeid · Accepted Answer · 2016-10-01 19:38:15Z

2

Your implementation of union-find is not correct. Consider this example

x -> y ( y is parent of x )

A -> B -> C
D -> E

When you call checkCycle( A, D) what should happen is all of the 5 nodes should go to one set, For example:

A -> B -> C
D -> E -> C

But what happens in your code is:

A -> B -> C
D -> C
E

Which is obviously not correct.

You can change the checkCycle as below:

static boolean checkCycle(int src, int dest) {

    int srcRoot = findParent(src);
    int destRoot = findParent(dest);
    if (srcRoot == destRoot ) {
        return true;
    }
    parent[destRoot] = srcRoot;
    return false;
}

I strongly advise you to read the wikipedia article about Disjoint-set and implement the path compression version, which improves the complexity.

answered Oct 1, 2016 at 19:38

Saeid

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1 Comment

sjsupersumit Over a year ago

Thanks. it indeed was bug in my union-find implementation.

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