我试图使用Undirected未加权列表图来实现Maze。但是我对java有困难。我得到空指针异常。我不知道如何实现这一目标。我已经花了很多时间，我想让这个工作，如果有人能给我指导，我将非常感激。
代码在这里。我没有使用任何已经实现的方法。我自己实现的大部分方法都是：
package maze3;
import java.util.Stack;
import java.util.LinkedList;
import java.util.NoSuchElementException;
import java.util.Hashtable;
import java.util.Stack;
// Single linked list
class SLLNode<E> {
protected E element;
protected SLLNode<E> succ;
public SLLNode(E elem, SLLNode<E> succ) {
this.element = elem;
this.succ = succ;
}
@Override
public String toString() {
return element.toString();
}
}
class GraphNode<E> {
private int index;
private E info;
private LinkedList<GraphNode<E>> neighbors;
public GraphNode(int index, E info) {
this.index = index;
this.info = info;
neighbors = new LinkedList<GraphNode<E>>();
}
boolean containsNeighbor(GraphNode<E> o) {
return neighbors.contains(o);
}
void addNeighbor(GraphNode<E> o) {
neighbors.add(o);
}
void removeNeighbor(GraphNode<E> o) {
if (neighbors.contains(o)) {
neighbors.remove(o);
}
}
@Override
public String toString() {
String ret = "INFO:" + info + " NEIGHBORS:";
for (int i = 0; i < neighbors.size(); i++) {
ret += neighbors.get(i).info + " ";
}
return ret;
}
@Override
public boolean equals(Object obj) {
@SuppressWarnings("unchecked")
GraphNode<E> pom = (GraphNode<E>) obj;
return (pom.info.equals(this.info));
}
public int getIndex() {
return index;
}
public void setIndex(int index) {
this.index = index;
}
public E getInfo() {
return info;
}
public void setInfo(E info) {
this.info = info;
}
public LinkedList<GraphNode<E>> getNeighbors() {
return neighbors;
}
public void setNeighbors(LinkedList<GraphNode<E>> neighbors) {
this.neighbors = neighbors;
}
}
interface Queue<E> {
public boolean isEmpty();
public int size();
public E peek();
public void clear();
public void enqueue(E x);
public E dequeue();
}
class LinkedQueue<E> implements Queue<E> {
SLLNode<E> front, rear;
int length;
public LinkedQueue() {
clear();
}
public boolean isEmpty() {
return (length == 0);
}
public int size() {
// Ja vrakja dolzinata na redicata.
return length;
}
public E peek() {
if (front == null) {
throw new NoSuchElementException();
}
return front.element;
}
public void clear() {
front = rear = null;
length = 0;
}
public void enqueue(E x) {
SLLNode<E> latest = new SLLNode<E>(x, null);
if (rear != null) {
rear.succ = latest;
rear = latest;
} else {
front = rear = latest;
}
length++;
}
public E dequeue() {
if (front != null) {
E frontmost = front.element;
front = front.succ;
if (front == null) {
rear = null;
}
length--;
return frontmost;
} else {
throw new NoSuchElementException();
}
}
}
// Graph implementation
class Graph<E> {
int num_nodes;
GraphNode<E> adjList[];
@SuppressWarnings("unchecked")
public Graph(int num_nodes, E[] list) {
this.num_nodes = num_nodes;
adjList = (GraphNode<E>[]) new GraphNode[num_nodes];
for (int i = 0; i < num_nodes; i++) {
adjList[i] = new GraphNode<E>(i, list[i]);
}
}
@SuppressWarnings("unchecked")
public Graph(int num_nodes) {
this.num_nodes = num_nodes;
adjList = (GraphNode<E>[]) new GraphNode[num_nodes];
}
int adjacent(int x, int y) {
return (adjList[x].containsNeighbor(adjList[y])) ? 1 : 0;
}
void addEdge(int x, int y) {
//System.out.println(adjList[x].getInfo());
if (!adjList[x].containsNeighbor(adjList[y])) {
adjList[x].addNeighbor(adjList[y]);
adjList[y].addNeighbor(adjList[x]);
}
}
void deleteEdge(int x, int y) {
adjList[x].removeNeighbor(adjList[y]);
adjList[y].removeNeighbor(adjList[x]);
}
public int getNum_nodes() {
return num_nodes;
}
void dfsSearch(int node) {
boolean visited[] = new boolean[num_nodes];
for (int i = 0; i < num_nodes; i++) {
visited[i] = false;
}
dfsRecursive(node, visited);
}
void dfsRecursive(int node, boolean visited[]) {
visited[node] = true;
System.out.println(node + ": " + adjList[node].getInfo());
for (int i = 0; i < adjList[node].getNeighbors().size(); i++) {
GraphNode<E> pom = adjList[node].getNeighbors().get(i);
if (!visited[pom.getIndex()]) {
dfsRecursive(pom.getIndex(), visited);
}
}
}
void dfsNonrecursive(int node) {
boolean visited[] = new boolean[num_nodes];
for (int i = 0; i < num_nodes; i++) {
visited[i] = false;
}
visited[node] = true;
System.out.println(node + ": " + adjList[node].getInfo());
Stack<Integer> s = new Stack<Integer>();
s.push(node);
GraphNode<E> pom;
while (!s.isEmpty()) {
pom = adjList[s.peek()];
GraphNode<E> tmp = null;
for (int i = 0; i < pom.getNeighbors().size(); i++) {
tmp = pom.getNeighbors().get(i);
if (!visited[tmp.getIndex()]) {
break;
}
}
if (tmp != null && !visited[tmp.getIndex()]) {
visited[tmp.getIndex()] = true;
System.out.println(tmp.getIndex() + ": " + tmp.getInfo());
s.push(tmp.getIndex());
} else {
s.pop();
}
}
}
void bfs(int node) {
boolean visited[] = new boolean[num_nodes];
for (int i = 0; i < num_nodes; i++) {
visited[i] = false;
}
visited[node] = true;
System.out.println(node + ": " + adjList[node].getInfo());
Queue<Integer> q = new LinkedQueue<Integer>();
q.enqueue(node);
GraphNode<E> pom;
while (!q.isEmpty()) {
pom = adjList[q.dequeue()];
GraphNode<E> tmp = null;
for (int i = 0; i < pom.getNeighbors().size(); i++) {
tmp = pom.getNeighbors().get(i);
if (!visited[tmp.getIndex()]) {
visited[tmp.getIndex()] = true;
System.out.println(tmp.getIndex() + ": " + tmp.getInfo());
q.enqueue(tmp.getIndex());
}
}
}
}
@Override
public String toString() {
String ret = new String();
for (int i = 0; i < this.num_nodes; i++) {
ret += i + ": " + adjList[i] + "\n";
}
return ret;
}
}
// MAIN MAZE CLASS
public class Maze3 {
/**
* Undirected unweighted graph with list
* The graph nodes are indexes
* The names of the nodes are translated into numbers with hash table
* Example 1,1 is translated to 1
*/
Graph g;
int start_node; //indeks temeto koe e vlez
int end_node; //indeks na temeto koe e izlez
Hashtable<String, Integer> h;
public Maze3() {
h = new Hashtable<String, Integer>();
}
void generateGraph(int rows, int columns, String[] in) {
int num_nodes = 0;
String key;
// Find start and end position
for (int i = 1; i < rows; i++) {
for (int j = 1; j < columns; j++) {
if (in[i].charAt(j) != '#') {
key = i + "," + j;
h.put(key, num_nodes);
if (in[i].charAt(j) == 'S') {
start_node = num_nodes;
}
if (in[i].charAt(j) == 'E') {
end_node = num_nodes;
}
num_nodes++;
}
}
}
g = new Graph(num_nodes);
int x;
int y;
//Generate the graph
for (int i = 1; i < rows; i++) {
for (int j = 1; j < columns; j++) {
if (in[i].charAt(j) != '#') {
//check if there is vertex before
if (in[i].charAt(j - 1) != '#') {
x = h.get(i + "," + j);
y = h.get(i + "," + (j - 1));
g.addEdge(x, y);
}
//check if there is vertex after
if (in[i].charAt(j + 1) != '#') {
x = h.get(i + "," + j);
y = h.get(i + "," + (j + 1));
g.addEdge(x, y);
}
//check if there is vertex on top
if (in[i - 1].charAt(j) != '#') {
x = h.get(i + "," + j);
y = h.get((i - 1) + "," + j);
g.addEdge(x, y);
}
//check if there is vertex on bottom
if (in[i + 1].charAt(j) != '#') {
x = h.get(i + "," + j);
y = h.get((i + 1) + "," + j);
g.addEdge(x, y);
}
}
}
}
}
void findPath() {
boolean visited[] = new boolean[g.getNum_nodes()];
for (int i = 0; i < g.getNum_nodes(); i++) {
visited[i] = false;
}
visited[start_node] = true;
Stack<Integer> s = new Stack<Integer>();
s.push(start_node);
int pom;
while (!s.isEmpty() && s.peek() != end_node) {
pom = s.peek();
int pom1 = pom;
for (int i = 0; i < g.getNum_nodes(); i++) {
if (g.adjacent(pom, i) == 1) {
pom1 = i;
if (!visited[i]) {
break;
}
}
}
if (!visited[pom1]) {
visited[pom1] = true;
//System.out.println(tmp.getIndex() + ": " + tmp.getInfo());
s.push(pom1);
} else {
s.pop();
}
}
if (s.isEmpty()) {
System.out.println("Nema reshenie");
} else {
Stack<Integer> os = new Stack<Integer>();
while (!s.isEmpty()) {
os.push(s.pop());
}
while (!os.isEmpty()) {
System.out.println(os.pop());
}
}
}
public static void main(String args[]) {
Maze3 m = new Maze3();
int rows = 6;
int columns = 6;
String[] in = new String[rows];
//6,6
in[0] = "######";
in[1] = "#S# E#";
in[2] = "# # ##";
in[3] = "# ##";
in[4] = "######";
in[5] = "######";
m.generateGraph(rows, columns, in);
System.out.println("Pateka:");
m.findPath();
}
}
函数addEdge（）中的错误，也许verexs是null但是如何初始化它们？用什么？
Exception in thread "main" java.lang.NullPointerException
at maze3.Graph.addEdge(Maze3.java:194)
at maze3.Maze3.generateGraph(Maze3.java:387)
at maze3.Maze3.main(Maze3.java:450)
Java Result: 1