给定一个无向图和一组顶点，我们必须使用广度优先搜索从给定的头节点打印所有不可达的节点。
例如：
Consider below undirected graph with two disconnected components:
In this graph, if we consider 0 as a head node, then the node 0, 1 and 2 are reachable. We mark all the reachable nodes as visited. All those nodes which are not marked as visited i.e, node 3 and 4 are non-reachable nodes.
例子：
Input: 5
0 1
0 2
1 2
3 4
Output: 3 4
Input: 8
0 1
0 2
1 2
3 4
4 5
6 7
Output: 3 4 5 6 7方法：
- 为此，我们可以使用BFS或DFS。本文的设置1实现了DFS方法。在本文中，将使用BFS方法。
- 我们从给定的来源进行BFS。由于给定的图是无向的，因此属于断开连接的组件的所有顶点都是不可访问的节点。为此，我们使用访问数组，该数组用于跟踪BFS中未访问的顶点。
- BFS是一种遍历算法，它从选定的节点(源节点或起始节点)开始遍历，并逐层遍历图，从而探索相邻节点(直接连接到源节点的节点)。然后，移至下一级邻居节点。
下面是上述方法的实现：
C++
// C++ program to count non-reachable nodes
// from a given source using BFS.
#include
using namespace std;
// Function to add an edge to graph
void add_edge(vector adj[],
int v, int w)
{
// Add w to v’s list.
adj[v].push_back(w);
// Add v to w's list.
adj[w].push_back(v);
}
// BFS traversal of the vertices
// reachable from starting node
void BFS(vector adj[], int s,
int v)
{
// Mark all the vertices
// as not visited
bool visited[v] = { false };
// Create a queue for BFS
queue q;
// Mark the current node as
// visited and enqueue it
q.push(s);
visited[s] = true;
while (!q.empty()) {
// Dequeue a vertex from
// queue
int p = q.front();
q.pop();
// Get all adjacent vertices
// of the dequeued vertex p.
// If a adjacent has not been
// visited, then mark it
// visited and enqueue it
for (auto it = adj[p].begin();
it != adj[p].end(); it++) {
if (!visited[*it]) {
visited[*it] = true;
q.push(*it);
}
}
}
for (int i = 0; i < v; i++) {
if (!visited[i]) {
cout << i << " ";
}
}
cout << "\n";
}
// Drivers code
int main()
{
// Create a graph given in
// the above diagram
vector adj[8];
add_edge(adj, 0, 1);
add_edge(adj, 0, 2);
add_edge(adj, 1, 2);
add_edge(adj, 3, 4);
add_edge(adj, 4, 5);
add_edge(adj, 6, 7);
BFS(adj, 0, 8);
return 0;
} Java
// Java program to count non-reachable nodes
// from a given source using BFS.
import java.util.*;
class GFG{
// Function to add an edge to graph
static void add_edge(Vector adj[],
int v, int w)
{
// Add w to v’s list.
adj[v].add(w);
// Add v to w's list.
adj[w].add(v);
}
// BFS traversal of the vertices
// reachable from starting node
static void BFS(Vector adj[], int s,
int v)
{
// Mark all the vertices
// as not visited
boolean []visited = new boolean[v];
// Create a queue for BFS
Queue q = new LinkedList<>();
// Mark the current node as
// visited and enqueue it
q.add(s);
visited[s] = true;
while (!q.isEmpty()) {
// Dequeue a vertex from
// queue
int p = q.peek();
q.remove();
// Get all adjacent vertices
// of the dequeued vertex p.
// If a adjacent has not been
// visited, then mark it
// visited and enqueue it
for (int it : adj[p]) {
if (!visited[it]) {
visited[it] = true;
q.add(it);
}
}
}
for (int i = 0; i < v; i++) {
if (!visited[i]) {
System.out.print(i+ " ");
}
}
System.out.print("\n");
}
// Drivers code
public static void main(String[] args)
{
// Create a graph given in
// the above diagram
Vector []adj = new Vector[8];
for (int i = 0; i < 8; i++)
adj[i] = new Vector();
add_edge(adj, 0, 1);
add_edge(adj, 0, 2);
add_edge(adj, 1, 2);
add_edge(adj, 3, 4);
add_edge(adj, 4, 5);
add_edge(adj, 6, 7);
BFS(adj, 0, 8);
}
}
// This code is contributed by sapnasingh4991 Python3
# Python3 program to count non-reachable
# nodes from a given source using BFS
# Function to add an edge to graph
def add_edge(adj, v, w):
# Add w to v’s list
adj[v].append(w)
# Add v to w's list
adj[w].append(v)
# BFS traversal of the vertices
# reachable from starting node
def BFS(adj, s, v):
# Mark all the vertices
# as not visited
visited = [False for i in range(v)]
# Create a queue for BFS
q = []
# Mark the current node as
# visited and enqueue it
q.append(s)
visited[s] = True
while (len(q) != 0):
# Dequeue a vertex from
# queue
p = q[0]
q.pop(0)
# Get all adjacent vertices
# of the dequeued vertex p.
# If a adjacent has not been
# visited, then mark it
# visited and enqueue it
for it in adj[p]:
if (not visited[it]):
visited[it] = True
q.append(it)
for i in range(v):
if (not visited[i]):
print(i, end = ' ')
print()
# Driver code
if __name__=='__main__':
# Create a graph given in
# the above diagram
adj = [[] for i in range(8)]
add_edge(adj, 0, 1)
add_edge(adj, 0, 2)
add_edge(adj, 1, 2)
add_edge(adj, 3, 4)
add_edge(adj, 4, 5)
add_edge(adj, 6, 7)
BFS(adj, 0, 8)
# This code is contributed by rutvik_56C#
// C# program to count non-reachable nodes
// from a given source using BFS.
using System;
using System.Collections.Generic;
class GFG{
// Function to add an edge to graph
static void add_edge(List []adj,
int v, int w)
{
// Add w to v’s list.
adj[v].Add(w);
// Add v to w's list.
adj[w].Add(v);
}
// BFS traversal of the vertices
// reachable from starting node
static void BFS(List []adj, int s,
int v)
{
// Mark all the vertices
// as not visited
bool []visited = new bool[v];
// Create a queue for BFS
List q = new List();
// Mark the current node as
// visited and enqueue it
q.Add(s);
visited[s] = true;
while (q.Count != 0) {
// Dequeue a vertex from
// queue
int p = q[0];
q.RemoveAt(0);
// Get all adjacent vertices
// of the dequeued vertex p.
// If a adjacent has not been
// visited, then mark it
// visited and enqueue it
foreach (int it in adj[p]) {
if (!visited[it]) {
visited[it] = true;
q.Add(it);
}
}
}
for (int i = 0; i < v; i++) {
if (!visited[i]) {
Console.Write(i + " ");
}
}
Console.Write("\n");
}
// Driver's code
public static void Main(String[] args)
{
// Create a graph given in
// the above diagram
List []adj = new List[8];
for (int i = 0; i < 8; i++)
adj[i] = new List();
add_edge(adj, 0, 1);
add_edge(adj, 0, 2);
add_edge(adj, 1, 2);
add_edge(adj, 3, 4);
add_edge(adj, 4, 5);
add_edge(adj, 6, 7);
BFS(adj, 0, 8);
}
}
// This code is contributed by 29AjayKumar 输出：
3 4 5 6 7