将无向连通图转换为强连通有向图

给定N个顶点和M 条边的无向图，任务是为给定的 M 条边分配方向，使该图成为强连通分量。如果图形不能转换为强连通分量，则打印“-1” 。

例子：

Input: N = 5, Edges[][] = { { 0, 1 }, { 0, 2 }, { 1, 2 }, { 1, 4 }, { 2, 3 }, { 3, 4 } } 
Output: 
0->1 
2->0 
4->1 
3->4 
2->3 
1->2 
Explanation: 
Below is the assigned edges to the above undirected graph: 
 

Input: N = 5, Edges[][] = { { 0, 1 }, { 0, 2 }, { 1, 3 }, { 2, 3 }, { 3, 4 } } 
Output: -1 
Explanation: 
Below is the graph for the above information: 
 

Since there is a bridge present in the above-undirected graph. Therefore, this graph can’t be converted into SCCs. 

方法：我们知道在任何有向图中都被称为在强连通分量(SCC)中，如果该图的所有顶点都是某个循环的一部分。当且仅当图中包含任何桥时，给定的无向图不会形成 SCC。以下是步骤：

• 我们将使用数组mark[]来存储 DFS 遍历期间的访问节点，使用order[]存储访问节点的索引号，以及使用bridge_detect[]存储给定图中存在的任何桥。
• 从顶点1开始 DFS 遍历。
• 遍历当前 Node 的 Adjacency 列表并执行以下操作：
  • 如果在任何 DFS 调用时再次遍历任何边，则忽略该边。
  • 如果子节点(节点 u )的顺序大于父节点(节点 v )的顺序，则忽略此当前边，因为Edges(v, u)已被处理。
  • 如果找到任何后边缘，则将当前父节点(节点 v )的桥边缘更新为：

bridge_detect[v] = min(order[u], bridge_detect[v]);

• 否则对当前子节点执行 DFS 遍历，并为当前节点重复步骤 3。
• 将当前节点的 DFS 调用后的网桥检测更新为：

bridge_detect[v] = min(bridge_detect[u], bridge_detect[v])

• 将当前的Edges(v, u) 对作为从节点 v 到节点 u 的有向边存储在一对数组中(比如arr[][] )。
• 如果给定图中存在任何桥接，则打印“-1” 。
• 否则打印存储在arr[][] 中的有向边。

下面是上述方法的实现：

C++

// C++ program for the above approach
 
#include 
using namespace std;
 
// To store the assigned Edges
vector > ans;
 
// Flag variable to check Bridges
int flag = 1;
 
// Function to implement DFS Traversal
int dfs(vector adj[],
        int* order, int* bridge_detect,
        bool* mark, int v, int l)
{
 
    // Mark the current node as visited
    mark[v] = 1;
 
    // Update the order of node v
    order[v] = order[l] + 1;
 
    // Update the bridge_detect for node v
    bridge_detect[v] = order[v];
 
    // Traverse the adjacency list of
    // Node v
    for (int i = 0; i < adj[v].size(); i++) {
        int u = adj[v][i];
 
        // Ignores if same edge is traversed
        if (u == l) {
            continue;
        }
 
        // Ignores the edge u --> v as
        // v --> u is already processed
        if (order[v] < order[u]) {
            continue;
        }
 
        // Finds a back Edges, cycle present
        if (mark[u]) {
 
            // Update the bridge_detect[v]
            bridge_detect[v]
                = min(order[u],
                      bridge_detect[v]);
        }
 
        // Else DFS traversal for current
        // node in the adjacency list
        else {
 
            dfs(adj, order, bridge_detect,
                mark, u, v);
        }
 
        // Update the bridge_detect[v]
        bridge_detect[v]
            = min(bridge_detect[u],
                  bridge_detect[v]);
 
        // Store the current directed Edge
        ans.push_back(make_pair(v, u));
    }
 
    // Condition for Bridges
    if (bridge_detect[v] == order[v]
        && l != 0) {
        flag = 0;
    }
 
    // Return flag
    return flag;
}
 
// Function to print the direction
// of edges to make graph SCCs
void convert(vector adj[], int n)
{
 
    // Arrays to store the visited,
    // bridge_detect and order of
    // Nodes
    int order[n] = { 0 };
    int bridge_detect[n] = { 0 };
    bool mark[n];
 
    // Intialise marks[] as false
    memset(mark, false, sizeof(mark));
 
    // DFS Traversal from vertex 1
    int flag = dfs(adj, order,
                   bridge_detect,
                   mark, 1, 0);
 
    // If flag is zero, then Bridge
    // is present in the graph
    if (flag == 0) {
        cout << "-1";
    }
 
    // Else print the direction of
    // Edges assigned
    else {
        for (auto& it : ans) {
            cout << it.first << "->"
                 << it.second << '\n';
        }
    }
}
 
// Function to create graph
void createGraph(int Edges[][2],
                 vector adj[],
                 int M)
{
 
    // Traverse the Edges
    for (int i = 0; i < M; i++) {
 
        int u = Edges[i][0];
        int v = Edges[i][1];
 
        // Push the edges in an
        // adjacency list
        adj[u].push_back(v);
        adj[v].push_back(u);
    }
}
 
// Driver Code
int main()
{
    // N vertices and M Edges
    int N = 5, M = 6;
    int Edges[M][2]
        = { { 0, 1 }, { 0, 2 },
            { 1, 2 }, { 1, 4 },
            { 2, 3 }, { 3, 4 } };
 
    // To create Adjacency List
    vector adj[N];
 
    // Create an undirected graph
    createGraph(Edges, adj, M);
 
    // Function Call
    convert(adj, N);
    return 0;
}

Java

// Java program for the above approach
import java.util.*;
import java.lang.*;
 
class GFG{
     
// To store the assigned Edges
static ArrayList ans;
  
// Flag variable to check Bridges
static int flag = 1;
  
// Function to implement DFS Traversal
static int dfs(ArrayList> adj,
               int[] order, int[] bridge_detect,
               boolean[] mark, int v, int l)
{
     
    // Mark the current node as visited
    mark[v] = true;
     
    // Update the order of node v
    order[v] = order[l] + 1;
  
    // Update the bridge_detect for node v
    bridge_detect[v] = order[v];
  
    // Traverse the adjacency list of
    // Node v
    for(int i = 0; i < adj.get(v).size(); i++)
    {
        int u = adj.get(v).get(i);
  
        // Ignores if same edge is traversed
        if (u == l)
        {
            continue;
        }
  
        // Ignores the edge u --> v as
        // v --> u is already processed
        if (order[v] < order[u])
        {
            continue;
        }
  
        // Finds a back Edges, cycle present
        if (mark[u])
        {
             
            // Update the bridge_detect[v]
            bridge_detect[v] = Math.min(order[u],
                                bridge_detect[v]);
        }
  
        // Else DFS traversal for current
        // node in the adjacency list
        else
        {
            dfs(adj, order, bridge_detect,
                mark, u, v);
        }
  
        // Update the bridge_detect[v]
        bridge_detect[v] = Math.min(bridge_detect[u],
                                    bridge_detect[v]);
  
        // Store the current directed Edge
        ans.add(new int[]{v, u});
    }
  
    // Condition for Bridges
    if (bridge_detect[v] == order[v] && l != 0)
    {
        flag = 0;
    }
     
    // Return flag
    return flag;
}
  
// Function to print the direction
// of edges to make graph SCCs
static void convert(ArrayList> adj,
                    int n)
{
     
    // Arrays to store the visited,
    // bridge_detect and order of
    // Nodes
    int[] order = new int[n];
    int[] bridge_detect = new int[n];
    boolean mark[] = new boolean[n];
     
    // DFS Traversal from vertex 1
    int flag = dfs(adj, order,
                   bridge_detect,
                   mark, 1, 0);
  
    // If flag is zero, then Bridge
    // is present in the graph
    if (flag == 0)
    {
        System.out.print("-1");
    }
  
    // Else print the direction of
    // Edges assigned
    else
    {
        for(int[] it : ans)
        {
            System.out.println(it[0] + "->" +
                               it[1]);
        }
    }
}
  
// Function to create graph
static void createGraph(int Edges[][],
                        ArrayList> adj,
                        int M)
{
     
    // Traverse the Edges
    for(int i = 0; i < M; i++)
    {
        int u = Edges[i][0];
        int v = Edges[i][1];
         
        // Push the edges in an
        // adjacency list
        adj.get(u).add(v);
        adj.get(v).add(u);
    }
}
 
// Driver code
public static void main(String[] args)
{
     
    // N vertices and M Edges
    int N = 5, M = 6;
     
    int Edges[][] = { { 0, 1 }, { 0, 2 },
                      { 1, 2 }, { 1, 4 },
                      { 2, 3 }, { 3, 4 } };
     
    // To create Adjacency List
    ArrayList> adj = new ArrayList<>();
    ans = new ArrayList<>();
     
    for(int i = 0; i < N; i++)
        adj.add(new ArrayList<>());
     
    // Create an undirected graph
    createGraph(Edges, adj, M);
     
    // Function Call
    convert(adj, N);
}
}
 
// This code is contributed by offbeat

Python3

# Python3 program for
# the above approach
 
# To store the assigned
# Edges
ans = []
  
# Flag variable to
# check Bridges
flag = 1;
  
# Function to implement
# DFS Traversal
def dfs(adj, order,
        bridge_detect,
        mark, v, l):
     
    global flag
     
    # Mark the current
    # node as visited
    mark[v] = 1;
  
    # Update the order of
    # node v
    order[v] = order[l] + 1;
  
    # Update the bridge_detect
    # for node v
    bridge_detect[v] = order[v];
  
    # Traverse the adjacency list of
    # Node v
    for i in range(len(adj[v])):       
        u = adj[v][i];
  
        # Ignores if same edge
        # is traversed
        if (u == l):
            continue;      
  
        # Ignores the edge u --> v as
        # v --> u is already processed
        if (order[v] < order[u]):
            continue;       
  
        # Finds a back Edges,
        # cycle present
        if (mark[u]):
  
            # Update the bridge_detect[v]
            bridge_detect[v] = min(order[u],
                                  bridge_detect[v]);
         
        # Else DFS traversal for current
        # node in the adjacency list
        else:
  
            dfs(adj, order,
                bridge_detect,
                mark, u, v);       
  
        # Update the bridge_detect[v]
        bridge_detect[v] = min(bridge_detect[u],
                              bridge_detect[v]);
  
        # Store the current
        # directed Edge
        ans.append([v, u]);
  
    # Condition for Bridges
    if (bridge_detect[v] ==
        order[v] and l != 0):
        flag = 0;
     
    # Return flag
    return flag;
  
# Function to print the
# direction of edges to
# make graph SCCs
def convert(adj, n):
  
    # Arrays to store the visited,
    # bridge_detect and order of
    # Nodes
    order = [0 for i in range(n)]
    bridge_detect = [0 for i in range(n)]
    mark = [False for i in range(n)]
  
    # DFS Traversal from
    # vertex 1
    flag = dfs(adj, order,
               bridge_detect,
               mark, 1, 0);
  
    # If flag is zero, then Bridge
    # is present in the graph
    if (flag == 0):
        print(-1)
  
    # Else print the direction
    # of Edges assigned
    else:
        for it in ans:
            print("{} -> {}".format(it[0],
                                    it[1]))
 
# Function to create graph
def createGraph(Edges,adj, M):
  
    # Traverse the Edges
    for i in range(M):
  
        u = Edges[i][0];
        v = Edges[i][1];
  
        # Push the edges in an
        # adjacency list
        adj[u].append(v);
        adj[v].append(u);
 
# Driver code
if __name__ == "__main__":
     
    # N vertices and M Edges
    N = 5
    M = 6;
    Edges = [[0, 1], [0, 2],
            [1, 2], [1, 4],
            [2, 3], [3, 4]];
  
    # To create Adjacency List
    adj = [[] for i in range(N)]
  
    # Create an undirected graph
    createGraph(Edges, adj, M);
  
    # Function Call
    convert(adj, N);
 
# This code is contributed by rutvik_56

C#

// C# program for the above approach
using System;
using System.Collections;
using System.Collections.Generic;
  
class GFG{
      
// To store the assigned Edges
static ArrayList ans;
   
// Flag variable to check Bridges
static int flag = 1;
   
// Function to implement DFS Traversal
static int dfs(ArrayList adj,
               int[] order, int[] bridge_detect,
               bool[] mark, int v, int l)
{
     
    // Mark the current node as visited
    mark[v] = true;
     
    // Update the order of node v
    order[v] = order[l] + 1;
     
    // Update the bridge_detect for node v
    bridge_detect[v] = order[v];
   
    // Traverse the adjacency list of
    // Node v
    for(int i = 0;
            i < ((ArrayList)adj[v]).Count;
            i++)
    {
        int u = (int)((ArrayList)adj[v])[i];
         
        // Ignores if same edge is traversed
        if (u == l)
        {
            continue;
        }
         
        // Ignores the edge u --> v as
        // v --> u is already processed
        if (order[v] < order[u])
        {
            continue;
        }
         
        // Finds a back Edges, cycle present
        if (mark[u])
        {
             
            // Update the bridge_detect[v]
            bridge_detect[v] = Math.Min(order[u],
                                bridge_detect[v]);
        }
         
        // Else DFS traversal for current
        // node in the adjacency list
        else
        {
            dfs(adj, order, bridge_detect,
                mark, u, v);
        }
   
        // Update the bridge_detect[v]
        bridge_detect[v] = Math.Min(bridge_detect[u],
                                    bridge_detect[v]);
   
        // Store the current directed Edge
        ans.Add(new int[]{v, u});
    }
   
    // Condition for Bridges
    if (bridge_detect[v] == order[v] && l != 0)
    {
        flag = 0;
    }
     
    // Return flag
    return flag;
}
   
// Function to print the direction
// of edges to make graph SCCs
static void convert(ArrayList adj,
                    int n)
{
     
    // Arrays to store the visited,
    // bridge_detect and order of
    // Nodes
    int[] order = new int[n];
    int[] bridge_detect = new int[n];
    bool []mark = new bool[n];
     
    // DFS Traversal from vertex 1
    int flag = dfs(adj, order,
                   bridge_detect,
                   mark, 1, 0);
                    
    // If flag is zero, then Bridge
    // is present in the graph
    if (flag == 0)
    {
        Console.Write("-1");
    }
     
    // Else print the direction of
    // Edges assigned
    else
    {
        foreach(int[] it in ans)
        {
            Console.WriteLine(it[0] + "->" +
                              it[1]);
        }
    }
}
   
// Function to create graph
static void createGraph(int [,]Edges,
                        ArrayList adj,
                        int M)
{
     
    // Traverse the Edges
    for(int i = 0; i < M; i++)
    {
        int u = Edges[i, 0];
        int v = Edges[i, 1];
         
        // Push the edges in an
        // adjacency list
        ((ArrayList)adj[u]).Add(v);
        ((ArrayList)adj[v]).Add(u);
    }
}
  
// Driver code
public static void Main(string[] args)
{
     
    // N vertices and M Edges
    int N = 5, M = 6;
      
    int [,]Edges = { { 0, 1 }, { 0, 2 },
                     { 1, 2 }, { 1, 4 },
                     { 2, 3 }, { 3, 4 } };
      
    // To create Adjacency List
    ArrayList adj = new ArrayList();
    ans = new ArrayList();
      
    for(int i = 0; i < N; i++)
        adj.Add(new ArrayList());
      
    // Create an undirected graph
    createGraph(Edges, adj, M);
      
    // Function Call
    convert(adj, N);
}
}
 
// This code is contributed by pratham76

输出：

0->1
2->0
4->1
3->4
2->3
1->2

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