Graph – Breath First Search

Below is a simple implementation of a graph and breath first search. The key is using a queue to store nodes.

1) Define a GraphNode

```class GraphNode{
int val;
GraphNode next;
GraphNode[] neighbors;
boolean visited;

GraphNode(int x) {
val = x;
}

GraphNode(int x, GraphNode[] n){
val = x;
neighbors = n;
}

public String toString(){
return "value: "+ this.val;
}
}
```

2) Define a Queue

```class Queue{
GraphNode first, last;

public void enqueue(GraphNode n){
if(first == null){
first = n;
last = first;
}else{
last.next = n;
last = n;
}
}

public GraphNode dequeue(){
if(first == null){
return null;
}else{
GraphNode temp = new GraphNode(first.val, first.neighbors);
first = first.next;
return temp;
}
}
}
```

3) Breath First Search uses a Queue

```public class GraphTest {

public static void main(String[] args) {
GraphNode n1 = new GraphNode(1);
GraphNode n2 = new GraphNode(2);
GraphNode n3 = new GraphNode(3);
GraphNode n4 = new GraphNode(4);
GraphNode n5 = new GraphNode(5);

n1.neighbors = new GraphNode[]{n2,n3,n5};
n2.neighbors = new GraphNode[]{n1,n4};
n3.neighbors = new GraphNode[]{n1,n4,n5};
n4.neighbors = new GraphNode[]{n2,n3,n5};
n5.neighbors = new GraphNode[]{n1,n3,n4};

breathFirstSearch(n1, 5);
}

public static void breathFirstSearch(GraphNode root, int x){
if(root.val == x)
System.out.println("find in root");

Queue queue = new Queue();
root.visited = true;
queue.enqueue(root);

while(queue.first != null){
GraphNode c = (GraphNode) queue.dequeue();
for(GraphNode n: c.neighbors){

if(!n.visited){
System.out.print(n + " ");
n.visited = true;
if(n.val == x)
System.out.println("Find "+n);
queue.enqueue(n);
}
}
}
}
}
```

Output:

value: 2 value: 3 value: 5 Find value: 5
value: 4