图的深度优先搜索或 DFS

图的深度优先遍历（或搜索）类似于树的深度优先遍历。这里唯一的问题是，与树不同，图可能包含循环（一个节点可能会被访问两次）。为避免多次处理节点，请使用布尔访问数组。

例子：

Input: n = 4, e = 6 
0 -> 1, 0 -> 2, 1 -> 2, 2 -> 0, 2 -> 3, 3 -> 3 
Output: DFS from vertex 1 : 1 2 0 3 
Explanation: 
DFS Diagram: 
 

Input: n = 4, e = 6 
2 -> 0, 0 -> 2, 1 -> 2, 0 -> 1, 3 -> 3, 1 -> 3 
Output: DFS from vertex 2 : 2 0 1 3 
Explanation: 
DFS Diagram: 
 

先决条件：
有关深度优先遍历的所有应用，请参阅此帖子。

方法：
深度优先搜索是一种用于遍历或搜索树或图数据结构的算法。该算法从根节点开始（在图的情况下选择某个任意节点作为根节点）并在回溯之前沿着每个分支尽可能地探索。所以基本思路是从根或任意一个节点开始，标记该节点，移动到相邻的未标记节点，继续这个循环，直到没有未标记的相邻节点。然后回溯并检查其他未标记的节点并遍历它们。最后，打印路径中的节点。

算法：
创建一个递归函数，该函数接受节点的索引和访问过的数组。

1. 将当前节点标记为已访问并打印该节点。
2. 遍历所有相邻和未标记的节点，并以相邻节点的索引调用递归函数。

执行：
下面是简单的深度优先遍历的实现。 C++ 实现使用图的邻接表表示。 STL 的列表容器用于存储相邻节点的列表。

C++

// C++ program to print DFS traversal from
// a given vertex in a  given graph
#include 
using namespace std;
 
// Graph class represents a directed graph
// using adjacency list representation
class Graph {
public:
    map visited;
    map > adj;
 
    // function to add an edge to graph
    void addEdge(int v, int w);
 
    // DFS traversal of the vertices
    // reachable from v
    void DFS(int v);
};
 
void Graph::addEdge(int v, int w)
{
    adj[v].push_back(w); // Add w to v’s list.
}
 
void Graph::DFS(int v)
{
    // Mark the current node as visited and
    // print it
    visited[v] = true;
    cout << v << " ";
 
    // Recur for all the vertices adjacent
    // to this vertex
    list::iterator i;
    for (i = adj[v].begin(); i != adj[v].end(); ++i)
        if (!visited[*i])
            DFS(*i);
}
 
// Driver code
int main()
{
    // Create a graph given in the above diagram
    Graph g;
    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 Depth First Traversal"
            " (starting from vertex 2) \n";
    g.DFS(2);
 
    return 0;
}
 
// improved by Vishnudev C

Java

// Java program to print DFS
// mtraversal from a given given
// graph
import java.io.*;
import java.util.*;
 
// This class represents a
// directed graph using adjacency
// list representation
class Graph {
    private int V; // No. of vertices
 
    // Array  of lists for
    // Adjacency List Representation
    private LinkedList adj[];
 
    // Constructor
    @SuppressWarnings("unchecked") 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); // Add w to v's list.
    }
 
    // A function used by DFS
    void DFSUtil(int v, boolean visited[])
    {
        // Mark the current node as visited and print it
        visited[v] = true;
        System.out.print(v + " ");
 
        // Recur for all the vertices adjacent to this
        // vertex
        Iterator i = adj[v].listIterator();
        while (i.hasNext()) {
            int n = i.next();
            if (!visited[n])
                DFSUtil(n, visited);
        }
    }
 
    // The function to do DFS traversal.
    // It uses recursive
    // DFSUtil()
    void DFS(int v)
    {
        // Mark all the vertices as
        // not visited(set as
        // false by default in java)
        boolean visited[] = new boolean[V];
 
        // Call the recursive helper
        // function to print DFS
        // traversal
        DFSUtil(v, visited);
    }
 
    // Driver Code
    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 Depth First Traversal "
            + "(starting from vertex 2)");
 
        g.DFS(2);
    }
}
// This code is contributed by Aakash Hasija

Python3

# Python3 program to print DFS traversal
# from a given given graph
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)
 
    # A function used by DFS
    def DFSUtil(self, v, visited):
 
        # Mark the current node as visited
        # and print it
        visited.add(v)
        print(v, end=' ')
 
        # Recur for all the vertices
        # adjacent to this vertex
        for neighbour in self.graph[v]:
            if neighbour not in visited:
                self.DFSUtil(neighbour, visited)
 
    # The function to do DFS traversal. It uses
    # recursive DFSUtil()
    def DFS(self, v):
 
        # Create a set to store visited vertices
        visited = set()
 
        # Call the recursive helper function
        # to print DFS traversal
        self.DFSUtil(v, visited)
 
# 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 DFS from (starting from vertex 2)")
g.DFS(2)
 
# This code is contributed by Neelam Yadav

C#

// C# program to print DFS traversal
// from a given graph
using System;
using System.Collections.Generic;
 
// This class represents a directed graph
// using adjacency list representation
class Graph {
    private int V; // No. of vertices
 
    // Array of lists for
    // Adjacency List Representation
    private List[] adj;
 
    // Constructor
    Graph(int v)
    {
        V = v;
        adj = new List[ v ];
        for (int i = 0; i < v; ++i)
            adj[i] = new List();
    }
 
    // Function to Add an edge into the graph
    void AddEdge(int v, int w)
    {
        adj[v].Add(w); // Add w to v's list.
    }
 
    // A function used by DFS
    void DFSUtil(int v, bool[] visited)
    {
        // Mark the current node as visited
        // and print it
        visited[v] = true;
        Console.Write(v + " ");
 
        // Recur for all the vertices
        // adjacent to this vertex
        List vList = adj[v];
        foreach(var n in vList)
        {
            if (!visited[n])
                DFSUtil(n, visited);
        }
    }
 
    // The function to do DFS traversal.
    // It uses recursive DFSUtil()
    void DFS(int v)
    {
        // Mark all the vertices as not visited
        // (set as false by default in c#)
        bool[] visited = new bool[V];
 
        // Call the recursive helper function
        // to print DFS traversal
        DFSUtil(v, visited);
    }
 
    // Driver Code
    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);
 
        Console.WriteLine(
            "Following is Depth First Traversal "
            + "(starting from vertex 2)");
 
        g.DFS(2);
        Console.ReadKey();
    }
}
 
// This code is contributed by techno2mahi

Javascript

 
// Javascript program to print DFS
// traversal from a given given
// graph
 
// This class represents a
// directed graph using adjacency
// list representation
class Graph
{
     
    // Constructor
    constructor(v)
    {
        this.V = v;
        this.adj = new Array(v);
        for(let i = 0; i < v; i++)
            this.adj[i] = [];
    }
     
    // Function to add an edge into the graph
    addEdge(v, w)
    {
         
        // Add w to v's list.
        this.adj[v].push(w);
    }
     
    // A function used by DFS
    DFSUtil(v, visited)
    {
         
        // Mark the current node as visited and print it
        visited[v] = true;
        document.write(v + " ");
  
        // Recur for all the vertices adjacent to this
        // vertex
        for(let i of this.adj[v].values())
        {
            let n = i
            if (!visited[n])
                this.DFSUtil(n, visited);
        }
    }
     
    // The function to do DFS traversal.
    // It uses recursive
    // DFSUtil()
    DFS(v)
    {
         
        // Mark all the vertices as
        // not visited(set as
        // false by default in java)
        let visited = new Array(this.V);
        for(let i = 0; i < this.V; i++)
            visited[i] = false;
  
        // Call the recursive helper
        // function to print DFS
        // traversal
        this.DFSUtil(v, visited);
    }
}
 
// Driver Code
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);
 
document.write("Following is Depth First Traversal " +
               "(starting from vertex 2)<br>");
 
g.DFS(2);
 
// This code is contributed by avanitrachhadiya2155
 

输出：

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

复杂性分析：

• 时间复杂度： O(V + E)，其中 V 是顶点数，E 是图中的边数。
• 空间复杂度： O(V)，因为需要一个额外访问的大小为 V 的数组。

处理断开连接的图：

• 解决方案：
  这将通过处理极端案例来实现。
  上面的代码仅遍历从给定源顶点可到达的顶点。从给定顶点可能无法到达所有顶点，如在断开连接图中。要对此类图进行完整的 DFS 遍历，请在 DFS 之后从所有未访问的节点运行 DFS。
  递归函数保持不变。
• 算法：
  1. 创建一个递归函数，该函数接受节点的索引和访问过的数组。
  2. 将当前节点标记为已访问并打印该节点。
  3. 遍历所有相邻和未标记的节点，并以相邻节点的索引调用递归函数。
  4. 从 0 到顶点数循环，检查该节点是否在之前的 DFS 中未被访问，用当前节点调用递归函数。

执行：

C++

// C++ program to print DFS
// traversal for a given given
// graph
#include 
using namespace std;
 
class Graph {
 
    // A function used by DFS
    void DFSUtil(int v);
 
public:
    map visited;
    map > adj;
    // function to add an edge to graph
    void addEdge(int v, int w);
 
    // prints DFS traversal of the complete graph
    void DFS();
};
 
void Graph::addEdge(int v, int w)
{
    adj[v].push_back(w); // Add w to v’s list.
}
 
void Graph::DFSUtil(int v)
{
    // Mark the current node as visited and print it
    visited[v] = true;
    cout << v << " ";
 
    // Recur for all the vertices adjacent to this vertex
    list::iterator i;
    for (i = adj[v].begin(); i != adj[v].end(); ++i)
        if (!visited[*i])
            DFSUtil(*i);
}
 
// The function to do DFS traversal. It uses recursive
// DFSUtil()
void Graph::DFS()
{
    // Call the recursive helper function to print DFS
    // traversal starting from all vertices one by one
    for (auto i : adj)
        if (visited[i.first] == false)
            DFSUtil(i.first);
}
 
// Driver  Code
int main()
{
    // Create a graph given in the above diagram
    Graph g;
    g.addEdge(0, 1);
    g.addEdge(0, 9);
    g.addEdge(1, 2);
    g.addEdge(2, 0);
    g.addEdge(2, 3);
    g.addEdge(9, 3);
 
    cout << "Following is Depth First Traversal \n";
    g.DFS();
 
    return 0;
}
// improved by Vishnudev C

Java

// Java program to print DFS
// traversal from a given given
// graph
import java.io.*;
import java.util.*;
 
// This class represents a
// directed graph using adjacency
// list representation
class Graph {
    private int V; // No. of vertices
 
    // Array  of lists for
    // Adjacency List Representation
    private LinkedList adj[];
 
    // Constructor
    @SuppressWarnings("unchecked") 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); // Add w to v's list.
    }
 
    // A function used by DFS
    void DFSUtil(int v, boolean visited[])
    {
        // Mark the current node as visited and print it
        visited[v] = true;
        System.out.print(v + " ");
 
        // Recur for all the vertices adjacent to this
        // vertex
        Iterator i = adj[v].listIterator();
        while (i.hasNext()) {
            int n = i.next();
            if (!visited[n])
                DFSUtil(n, visited);
        }
    }
 
    // The function to do DFS traversal. It uses recursive
    // DFSUtil()
    void DFS()
    {
        // Mark all the vertices as not visited(set as
        // false by default in java)
        boolean visited[] = new boolean[V];
 
        // Call the recursive helper function to print DFS
        // traversal starting from all vertices one by one
        for (int i = 0; i < V; ++i)
            if (visited[i] == false)
                DFSUtil(i, visited);
    }
 
    // Driver Code
    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 Depth First Traversal");
 
        g.DFS();
    }
}
// This code is contributed by Aakash Hasija

Python3

'''Python program to print DFS traversal for complete graph'''
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)
    # A function used by DFS
 
    def DFSUtil(self, v, visited):
        # Mark the current node as visited and print it
        visited.add(v)
        print(v,end=" ")
 
        # recur for all the vertices adjacent to this vertex
        for neighbour in self.graph[v]:
            if neighbour not in visited:
                self.DFSUtil(neighbour, visited)
        # The function to do DFS traversal. It uses recursive DFSUtil
 
    def DFS(self):
        # create a set to store all visited vertices
        visited = set()
        # call the recursive helper function to print DFS traversal starting from all
        # vertices one by one
        for vertex in self.graph:
            if vertex not in visited:
                self.DFSUtil(vertex, visited)
# Driver code
# create a graph given in the above diagram
 
print("Following is Depth First Traversal \n")
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)
g.DFS()
 
# Improved by Dheeraj Kumar

C＃

// C# program to print DFS
// traversal from a given given
// graph
using System;
using System.Collections.Generic;
 
// This class represents a
// directed graph using adjacency
// list representation
public class Graph {
    private int V; // No. of vertices
 
    // Array of lists for
    // Adjacency List Representation
    private List[] adj;
 
    // Constructor
    Graph(int v)
    {
        V = v;
        adj = new List[ v ];
        for (int i = 0; i < v; ++i)
            adj[i] = new List();
    }
 
    // Function to add an edge into the graph
    void addEdge(int v, int w)
    {
        adj[v].Add(w); // Add w to v's list.
    }
 
    // A function used by DFS
    void DFSUtil(int v, bool[] visited)
    {
        // Mark the current
        // node as visited and print it
        visited[v] = true;
        Console.Write(v + " ");
 
        // Recur for all the
        // vertices adjacent to this
        // vertex
        foreach(int i in adj[v])
        {
            int n = i;
            if (!visited[n])
                DFSUtil(n, visited);
        }
    }
 
    // The function to do
    // DFS traversal. It uses recursive
    // DFSUtil()
    void DFS()
    {
        // Mark all the vertices as not visited(set as
        // false by default in java)
        bool[] visited = new bool[V];
 
        // Call the recursive helper
        // function to print DFS
        // traversal starting from
        // all vertices one by one
        for (int i = 0; i < V; ++i)
            if (visited[i] == false)
                DFSUtil(i, visited);
    }
 
    // Driver code
    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);
 
        Console.WriteLine(
            "Following is Depth First Traversal");
 
        g.DFS();
    }
}
 
// This code is contributed by PrinciRaj1992

Javascript

      // JavaScript program to print DFS
      // traversal from a given given
      // graph
 
      // This class represents a
      // directed graph using adjacency
      // list representation
      class Graph
      {
       
        // Constructor
        constructor(v) {
          this.V = v;
          this.adj = new Array(v).fill([]);
        }
 
        // Function to Add an edge into the graph
        AddEdge(v, w) {
          this.adj[v].push(w); // Add w to v's list.
        }
 
        // A function used by DFS
        DFSUtil(v, visited)
        {
         
          // Mark the current
          // node as visited and print it
          visited[v] = true;
          document.write(v + " ");
 
          // Recur for all the
          // vertices adjacent to this
          // vertex
          for (const n of this.adj[v]) {
            if (!visited[n]) this.DFSUtil(n, visited);
          }
        }
 
        // The function to do
        // DFS traversal. It uses recursive
        // DFSUtil()
        DFS()
        {
         
          // Mark all the vertices as not visited(set as
          var visited = new Array(this.V).fill(false);
 
          // Call the recursive helper
          // function to print DFS
          // traversal starting from
          // all vertices one by one
          for (var i = 0; i < this.V; ++i)
            if (visited[i] == false) this.DFSUtil(i, visited);
        }
      }
       
      // Driver Code
      var 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);
 
      document.write("Following is Depth First Traversal<br>");
 
      g.DFS();
       
      // This code is contributed by rdtank.
    

输出：

Following is Depth First Traversal
0 1 2 3 9

复杂性分析：

• 时间复杂度： O(V + E)，其中 V 是顶点数，E 是图中的边数。
• 空间复杂度： O(V)，因为需要一个额外访问的大小为 V 的数组。

https://youtu.be/Y40bRyPQQr0

• DFS 的应用。
• 图的广度优先遍历
• 最近关于 DFS 的文章