图论算法：图的广度优先搜索或BFS解析和代码实现

2021年3月9日16:16:39 发表评论 728 次浏览

本文概述

广度优先遍历(或搜索)

图的类似于树的广度优先遍历(请参见方法2

这个帖子

)。

唯一的问题是, 与树不同, 图可能包含循环, 因此我们可能会再次来到同一节点。为了避免多次处理节点, 我们使用布尔访问数组。为了简单起见, 假设所有顶点都可以从起始顶点到达。

例如, 在下图中, 我们从顶点2开始遍历。当到达顶点0时, 我们将寻找它的所有相邻顶点。 2也是0的相邻顶点。如果我们不标记访问的顶点, 那么2将再次被处理, 它将成为一个非终止过程。下图的广度优先遍历为2、0、3、1。

图形的广度优先搜索或BFS1

推荐：请在"实践首先, 在继续解决方案之前。

以下是来自给定源的简单"广度优先遍历"的实现。

实现使用邻接表表示图。STL‘s列出容器用于存储BFS遍历所需的相邻节点列表和节点队列。

C ++

// Program to print BFS traversal from a given
// source vertex. BFS(int s) traverses vertices 
// reachable from s.
#include<iostream>
#include <list>
  
using namespace std;
  
// This class represents a directed graph using
// adjacency list representation
class Graph
{
     int V;    // No. of vertices
  
     // Pointer to an array containing adjacency
     // lists
     list< int > *adj;   
public :
     Graph( int V);  // Constructor
  
     // function to add an edge to graph
     void addEdge( int v, int w); 
  
     // prints BFS traversal from a given source s
     void BFS( int s);  
};
  
Graph::Graph( int V)
{
     this ->V = V;
     adj = new list< int >[V];
}
  
void Graph::addEdge( int v, int w)
{
     adj[v].push_back(w); // Add w to v’s list.
}
  
void Graph::BFS( int s)
{
     // Mark all the vertices as not visited
     bool *visited = new bool [V];
     for ( int i = 0; i < V; i++)
         visited[i] = false ;
  
     // Create a queue for BFS
     list< int > queue;
  
     // Mark the current node as visited and enqueue it
     visited展开 = true ;
     queue.push_back(s);
  
     // 'i' will be used to get all adjacent
     // vertices of a vertex
     list< int >::iterator i;
  
     while (!queue.empty())
     {
         // Dequeue a vertex from queue and print it
         s = queue.front();
         cout << s << " " ;
         queue.pop_front();
  
         // Get all adjacent vertices of the dequeued
         // vertex s. If a adjacent has not been visited, // then mark it visited and enqueue it
         for (i = adj展开.begin(); i != adj展开.end(); ++i)
         {
             if (!visited[*i])
             {
                 visited[*i] = true ;
                 queue.push_back(*i);
             }
         }
     }
}
  
// Driver program to test methods of graph class
int main()
{
     // Create a graph given in the above diagram
     Graph g(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);
  
     cout << "Following is Breadth First Traversal "
          << "(starting from vertex 2) \n" ;
     g.BFS(2);
  
     return 0;
}

Java

// Java program to print BFS traversal from a given source vertex.
// BFS(int s) traverses vertices reachable from s.
import java.io.*;
import java.util.*;
  
// This class represents a directed graph using adjacency list
// representation
class Graph
{
     private int V;   // No. of vertices
     private LinkedList<Integer> adj[]; //Adjacency Lists
  
     // Constructor
     Graph( int v)
     {
         V = v;
         adj = new LinkedList[v];
         for ( int i= 0 ; i<v; ++i)
             adj[i] = new LinkedList();
     }
  
     // Function to add an edge into the graph
     void addEdge( int v, int w)
     {
         adj[v].add(w);
     }
  
     // prints BFS traversal from a given source s
     void BFS( int s)
     {
         // Mark all the vertices as not visited(By default
         // set as false)
         boolean visited[] = new boolean [V];
  
         // Create a queue for BFS
         LinkedList<Integer> queue = new LinkedList<Integer>();
  
         // Mark the current node as visited and enqueue it
         visited展开= true ;
         queue.add(s);
  
         while (queue.size() != 0 )
         {
             // Dequeue a vertex from queue and print it
             s = queue.poll();
             System.out.print(s+ " " );
  
             // Get all adjacent vertices of the dequeued vertex s
             // If a adjacent has not been visited, then mark it
             // visited and enqueue it
             Iterator<Integer> i = adj展开.listIterator();
             while (i.hasNext())
             {
                 int n = i.next();
                 if (!visited[n])
                 {
                     visited[n] = true ;
                     queue.add(n);
                 }
             }
         }
     }
  
     // Driver method to
     public static void main(String args[])
     {
         Graph g = new Graph( 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 );
     }
}
// This code is contributed by Aakash Hasija

Python3

# Python3 Program to print BFS traversal
# from a given source vertex. BFS(int s)
# traverses vertices reachable from s.
from collections import defaultdict
  
# This class represents a directed graph
# using adjacency list representation
class Graph:
  
     # Constructor
     def __init__( self ):
  
         # default dictionary to store graph
         self .graph = defaultdict( list )
  
     # function to add an edge to graph
     def addEdge( self , u, v):
         self .graph[u].append(v)
  
     # Function to print a BFS of graph
     def BFS( self , s):
  
         # Mark all the vertices as not visited
         visited = [ False ] * ( len ( self .graph))
  
         # Create a queue for BFS
         queue = []
  
         # Mark the source node as 
         # visited and enqueue it
         queue.append(s)
         visited展开 = True
  
         while queue:
  
             # Dequeue a vertex from 
             # queue and print it
             s = queue.pop( 0 )
             print (s, end = " " )
  
             # Get all adjacent vertices of the
             # dequeued vertex s. If a adjacent
             # has not been visited, then mark it
             # visited and enqueue it
             for i in self .graph展开:
                 if visited[i] = = False :
                     queue.append(i)
                     visited[i] = True
  
# Driver code
  
# Create a graph given in
# the above diagram
g = Graph()
g.addEdge( 0 , 1 )
g.addEdge( 0 , 2 )
g.addEdge( 1 , 2 )
g.addEdge( 2 , 0 )
g.addEdge( 2 , 3 )
g.addEdge( 3 , 3 )
  
print ( "Following is Breadth First Traversal"
                   " (starting from vertex 2)" )
g.BFS( 2 )
  
# This code is contributed by Neelam Yadav

C#

// C# program to print BFS traversal 
// from a given source vertex. 
// BFS(int s) traverses vertices 
// reachable from s. 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
  
// This class represents a directed 
// graph using adjacency list 
// representation 
class Graph{
      
// No. of vertices     
private int _V; 
  
//Adjacency Lists 
LinkedList< int >[] _adj; 
  
public Graph( int V)
{
     _adj = new LinkedList< int >[V];
     for ( int i = 0; i < _adj.Length; i++)
     {
         _adj[i] = new LinkedList< int >();
     }
     _V = V;
}
  
// Function to add an edge into the graph 
public void AddEdge( int v, int w)
{         
     _adj[v].AddLast(w);
  
}
  
// Prints BFS traversal from a given source s 
public void BFS( int s)
{
      
     // Mark all the vertices as not
     // visited(By default set as false) 
     bool [] visited = new bool [_V];
     for ( int i = 0; i < _V; i++)
         visited[i] = false ;
      
     // Create a queue for BFS 
     LinkedList< int > queue = new LinkedList< int >();
      
     // Mark the current node as 
     // visited and enqueue it 
     visited展开 = true ;
     queue.AddLast(s);         
  
     while (queue.Any())
     {
          
         // Dequeue a vertex from queue 
         // and print it
         s = queue.First();
         Console.Write(s + " " );
         queue.RemoveFirst();
          
         // Get all adjacent vertices of the 
         // dequeued vertex s. If a adjacent
         // has not been visited, then mark it 
         // visited and enqueue it 
         LinkedList< int > list = _adj展开;
  
         foreach ( var val in list)             
         {
             if (!visited[val])
             {
                 visited[val] = true ;
                 queue.AddLast(val);
             }
         }
     }
}
  
// Driver code
static void Main( string [] args)
{
     Graph g = new Graph(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);
      
     Console.Write( "Following is Breadth First " +
                   "Traversal(starting from " +
                   "vertex 2)\n" );
     g.BFS(2);
}
}
  
// This code is contibuted by anv89

输出如下：

Following is Breadth First Traversal (starting from vertex 2)
2 0 3 1

插图：

bfs1
bfs2
bfs3
bfs4
bfs6
bfs7
bfs8
bfs9
bfs10
bfs11

请注意, 以上代码仅遍历给定源顶点可到达的顶点。从给定的顶点可能无法到达所有顶点(例如, 断开连接的图)。要打印所有顶点, 我们可以修改BFS函数以从所有节点开始一一遍历遍历(例如DFS修改版)。

时间复杂度：O(V + E), 其中V是图形中顶点的数量, E是图形中边的数量。

你可能还希望在下面看到：

  • 关于BFS的最新文章
  • 深度优先遍历
  • 广度优先遍历的应用
  • 深度优先搜索的应用

如果发现任何不正确的地方, 或者想分享有关上述主题的更多信息, 请写评论。

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