Breadth First Traversal (or Search) for a graph may contain cycles, so we may come to the same node again. To avoid processing a node more than once, we use a boolean visited array. For simplicity, it is assumed that all vertices are reachable from the starting vertex.

广度优先遍历(或搜索)图像可能包含周期,所以我们可能会再次相同的节点。为了避免处理节点不止一次,我们使用一个布尔访问数组。为简单起见,假设所有顶点都可以从顶点开始。

For example, in the following graph, we start traversal from vertex 2. When we come to vertex 0, we look for all adjacent vertices of it. 2 is also an adjacent vertex of 0. If we don’t mark visited vertices, then 2 will be processed again and it will become a non-terminating process. A Breadth First Traversal of the following graph is 2, 0, 3, 1.

例如,在下面的图,我们从顶点开始遍历2。当我们来到顶点0,我们寻找所有相邻顶点。2也是一个相邻的顶点0。如果我们不标记访问顶点,然后再处理一遍,它将成为一个终止过程。以下图的广度优先遍历2 0 3 1。

广度优先遍历

import java.util.LinkedList;public class BreadthFirstSearchGraph{    private int V;    private LinkedList<Integer>[] adj;    public BreadthFirstSearchGraph(int v){        this.V = v;        adj = new LinkedList[V];        for (int i = 0; i < V ; i++) {            adj[i] = new LinkedList<>();        }    }    public void addEdge(int v, int w){        adj[v].add(w);    }    public void bfs(int s) {        boolean[] visited = new boolean[V];        LinkedList<Integer> queue = new LinkedList<>();        visited[s] = true;        queue.add(s);        while(queue.size() != 0){            int r = queue.poll();            System.out.println(" " + r + " ");            for (Integer item : adj[r]) {                if (!visited[item]){                    visited[item] = true;                    queue.add(item);                }            }        }    }    public static void main(String[] args) {        BreadthFirstSearchGraph g = new BreadthFirstSearchGraph(4);        g.addEdge(0, 1);        g.addEdge(0, 2);        g.addEdge(1, 2);        g.addEdge(2, 0);        g.addEdge(2, 3);        g.addEdge(3, 3);        System.out.println("Following is Breadth First Traversal "+                "(starting from vertex 2)");        g.bfs(2);    }}