getting nzec.. plz help...

I tried to run this program for the primedistance question asked in august 13 challenge.. I just visited every node and checked it's distance from every other node.. it's giving correct answers for some test cases that I tried but on submitting it, it's showing nzec error.. help plz.. here's the code..

 class CLASSNAME {
    static long total=0;
    public void solve(int testNumber, InputReader in, OutputWriter out) {
        int n=in.readInt();
        boolean[][] tree=new boolean[n+1][n+1];
        boolean[] isPrime=IntegerUtils.generatePrimalityTable(100001);
        boolean[] isVis;
        for(int i=1;i<n;i++){
            int a=in.readInt();int b=in.readInt();
            tree[a][b]=tree[b][a]=true;
        }

        for(int i=1;i<=n;i++){
            isVis=new boolean[n+1];
            recurse(tree,isPrime,isVis,1,i);
        }
        //out.printLine(total);
        out.printLine((total/2)/((n*(n-1))/2.0));
    }
    static void recurse(boolean[][] tree,boolean[] isPrime,boolean[] isVis,int sum,int level){
        if(isVis[level])return;
        isVis[level]=true;
        //if(level>=tree[0].length)return;
        for(int i=1;i<tree[0].length;i++){
            if(tree[level][i]){
                if(isPrime[sum] && !isVis[i]){
                    total++;
                //System.out.println(level+" "+i+" "+sum);
                }
                recurse(tree,isPrime,isVis,sum+1,i);
            }
        }
    }
}