Searching bridge in undirected graph

Question

Hi,
we have a homework to school to find a bridge in undirected graph.
We got input:
8 - number of nodes
0 3 - node 0 is connected with node 3
1 3
2 3
4 5
4 6
4 7
3 4
0 1
1 2
5 6
6 7
0 0 - end of input
Here is a correct output for this graph:
3 4 - edge from node 3 to node 4 is a bridge
0 0 - end of output
Here is my code which work perfectly for any graphs i think about but when i tried to commit it to our evaluating system it fails in 5 cases from 10. System says that it fails on diamond graph without needlepoint. I think I must have something wrong when i go throught graph (some condition or something). I tried to implement algorithm described here [http://www.cs.ucr.edu/~neal/2005/cs141/wiki/index.cgi?Cs141_Home/Lecture10] So if you know whats wrong or you have some idea please help.:)
package pal;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;
class Node{
enum NodeState {
White, Gray, Black
NodeState state = NodeState.White;
int id = 0;
int hm = 0;
int zm = 0;
int from = -1;
boolean root = false;
ArrayList<Node> children = new ArrayList<Node>();
public int min(int hm, int zm){
int min;
if(hm < zm){
min = hm;
return min;}
else{
min = zm;
return min;
public void tree(){
if(state == NodeState.White){
state = NodeState.Gray;
for (int i = 0; i < children.size(); i ){
if (children.get(i).state == NodeState.Black){ continue;}
else if (children.get(i).state == NodeState.Gray && children.get(i).id == from){continue;}
else if(children.get(i).state == NodeState.Gray && children.get(i).id != from){
zm = min(children.get(i).hm,zm);
continue;
else{
children.get(i).from = id;
children.get(i).hm = hm  +1;+
+children.get(i).zm = children.get(i).hm;+
+children.get(i).tree();}+
+}+
+if(state != NodeState.Black && root){+
+state = NodeState.Black;+
+}+
+else if(!root && state != NodeState.Black){+
+state = NodeState.Black;+
+for (int i = 0; i< children.size();i ++){
if(children.get(i).id == from){
children.get(i).zm = min(children.get(i).zm, zm);
children.get(i).tree();
public class Main {
public static void main(String[] args) throws IOException {
Node n;
int node1, node2;
int non;
ArrayList<Node> nodeList = new ArrayList<Node>();
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
non = Integer.parseInt(br.readLine());
for(int i = 0; i < non; i++){
n = new Node();
n.id = i;
if(i==0){
n.root = true;
nodeList.add(n);
while(true){
StringTokenizer st = new StringTokenizer(br.readLine());
node1 = Integer.parseInt(st.nextToken());
node2 = Integer.parseInt(st.nextToken());
if( (node1+node2) == 0) break;
nodeList.get(node1).children.add(nodeList.get(node2));
nodeList.get(node2).children.add(nodeList.get(node1));
/*for(int i = 0; i < non ; i++){*
*nodeList.get(i).tree();*
nodeList.get(0).tree();
// System.out.println("--------------");
StringBuilder sb = new StringBuilder();
for (int i = 0; i < non; i++){
if((nodeList.get(i).zm == nodeList.get(i).hm)&& !nodeList.get(i).root){
sb.append(nodeList.get(i).from);
sb.append(" ");
sb.append(nodeList.get(i).id);
sb.append("\n");
sb.append("0 0");
System.out.println(sb);

Answer 1

xtem wrote:
Hi,
we have a homework to school to find a bridge in undirected graph.
We got input:
8 - number of nodes
0 3 - node 0 is connected with node 3
1 3
2 3
4 5
4 6
4 7
3 4
0 1
1 2
5 6
6 7
0 0 - end of input
Here is a correct output for this graph:
3 4 - edge from node 3 to node 4 is a bridge
0 0 - end of output
Here is my code which work perfectly for any graphs i think about but when i tried to commit it to our evaluating system it fails in 5 cases from 10. System says that it fails on diamond graph without needlepoint. I think I must have something wrong when i go throught graph (some condition or something). I tried to implement algorithm described here [http://www.cabletrain.com] So if you know whats wrong or you have some idea please help.:)
package pal;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;
class Node{
enum NodeState {
White, Gray, Black
NodeState state = NodeState.White;
int id = 0;
int hm = 0;
int zm = 0;
int from = -1;
boolean root = false;
ArrayList<Node> children = new ArrayList<Node>();
public int min(int hm, int zm){
int min;
if(hm < zm){
min = hm;
return min;}
else{
min = zm;
return min;
public void tree(){
if(state == NodeState.White){
state = NodeState.Gray;
for (int i = 0; i < children.size(); i ){
if (children.get(i).state == NodeState.Black){ continue;}
else if (children.get(i).state == NodeState.Gray && children.get(i).id == from){continue;}
else if(children.get(i).state == NodeState.Gray && children.get(i).id != from){
zm = min(children.get(i).hm,zm);
continue;
else{
children.get(i).from = id;
children.get(i).hm = hm  +1;+
+children.get(i).zm = children.get(i).hm;+
+children.get(i).tree();}+
+}+
+if(state != NodeState.Black && root){+
+state = NodeState.Black;+
+}+
+else if(!root && state != NodeState.Black){+
+state = NodeState.Black;+
+for (int i = 0; i< children.size();i ++){
if(children.get(i).id == from){
children.get(i).zm = min(children.get(i).zm, zm);
children.get(i).tree();
public class Main {
public static void main(String[] args) throws IOException {
Node n;
int node1, node2;
int non;
ArrayList<Node> nodeList = new ArrayList<Node>();
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
non = Integer.parseInt(br.readLine());
for(int i = 0; i < non; i++){
n = new Node();
n.id = i;
if(i==0){
n.root = true;
nodeList.add(n);
while(true){
StringTokenizer st = new StringTokenizer(br.readLine());
node1 = Integer.parseInt(st.nextToken());
node2 = Integer.parseInt(st.nextToken());
if( (node1+node2) == 0) break;
nodeList.get(node1).children.add(nodeList.get(node2));
nodeList.get(node2).children.add(nodeList.get(node1));
/*for(int i = 0; i < non ; i++){*
*nodeList.get(i).tree();*
nodeList.get(0).tree();
// System.out.println("--------------");
StringBuilder sb = new StringBuilder();
for (int i = 0; i < non; i++){
if((nodeList.get(i).zm == nodeList.get(i).hm)&& !nodeList.get(i).root){
sb.append(nodeList.get(i).from);
sb.append(" ");
sb.append(nodeList.get(i).id);
sb.append("\n");
sb.append("0 0");
System.out.println(sb);