📜  形成三角形需要增加的最小边数

📅  最后修改于: 2021-06-26 23:07:10             🧑  作者: Mango

给定具有N个顶点和N个边的无向图。没有两个边连接同一对顶点。三角形是一组三个不同的顶点,这样,每对顶点通过一条边连接,即,如果图形包含边(u,v)(v,w) ,则三个不同的顶点uvw是三角形和(v,u)
任务是找到需要添加到给定图中的最小边数,以使该图包含至少一个三角形。

例子:

Input:
  1
 / \
2   3
Output: 1

Input:
  1     3
 /     /
2     4
Output: 2

方法:初始化ans = 3 ,这是形成三角形所需的最大边数。现在,对于每个可能的顶点三元组,有四种情况:

  1. 情况1:如果存在节点ijk ,使得(i,j)(j,k)(k,i)有一条边则答案为0
  2. 情况2:如果存在节点ijk使得仅连接了两对顶点,则需要一个边来形成三角形。所以更新ans = min(ans,1)
  3. 情况3:否则,如果仅连接一对顶点,则ans = min(ans,2)
  4. 情况4:如果没有边,则ans = min(ans,3)

最后打印ans

下面是上述方法的实现。

C++
// C++ implementation of the approach
#include 
using namespace std;
  
// Function to return the minimum number
// of edges that need to be added to
// the given graph such that it
// contains at least one triangle
int minEdges(vector > v, int n)
{
  
    // adj is the adjacency matrix such that
    // adj[i][j] = 1 when there is an
    // edge between i and j
    vector > adj;
    adj.resize(n + 1);
    for (int i = 0; i < adj.size(); i++)
        adj[i].resize(n + 1, 0);
  
    // As the graph is undirected
    // so there will be an edge
    // between (i, j) and (j, i)
    for (int i = 0; i < v.size(); i++) {
        adj[v[i].first][v[i].second] = 1;
        adj[v[i].second][v[i].first] = 1;
    }
  
    // To store the required
    // count of edges
    int edgesNeeded = 3;
  
    // For every possible vertex triplet
    for (int i = 1; i <= n; i++) {
        for (int j = i + 1; j <= n; j++) {
            for (int k = j + 1; k <= n; k++) {
  
                // If the vertices form a triangle
                if (adj[i][j] && adj[j][k] && adj[k][i])
                    return 0;
  
                // If no edges are present
                if (!(adj[i][j] || adj[j][k] || adj[k][i]))
                    edgesNeeded = min(edgesNeeded, 3);
  
                else {
  
                    // If only 1 edge is required
                    if ((adj[i][j] && adj[j][k])
                        || (adj[j][k] && adj[k][i])
                        || (adj[k][i] && adj[i][j])) {
                        edgesNeeded = 1;
                    }
  
                    // Two edges are required
                    else
                        edgesNeeded = min(edgesNeeded, 2);
                }
            }
        }
    }
    return edgesNeeded;
}
  
// Driver code
int main()
{
  
    // Number of nodes
    int n = 3;
  
    // Storing the edges in a vector of pairs
    vector > v = { { 1, 2 }, { 1, 3 } };
  
    cout << minEdges(v, n);
  
    return 0;
}


Java
// Java implementation of the approach
import java.io.*;
import java.util.*;
  
class Pair 
{
    V first;
    E second;
  
    Pair(V first, E second) 
    {
        this.first = first;
        this.second = second;
    }
}
  
class GFG 
{
  
    // Function to return the minimum number
    // of edges that need to be added to
    // the given graph such that it
    // contains at least one triangle
    static int minEdges(Vector> v, int n) 
    {
  
        // adj is the adjacency matrix such that
        // adj[i][j] = 1 when there is an
        // edge between i and j
        int[][] adj = new int[n + 1][n + 1];
  
        // As the graph is undirected
        // so there will be an edge
        // between (i, j) and (j, i)
        for (int i = 0; i < v.size(); i++) 
        {
            adj[v.elementAt(i).first]
               [v.elementAt(i).second] = 1;
            adj[v.elementAt(i).second]
               [v.elementAt(i).first] = 1;
        }
  
        // To store the required
        // count of edges
        int edgesNeeded = 0;
  
        // For every possible vertex triplet
        for (int i = 1; i <= n; i++)
        {
            for (int j = i + 1; j <= n; j++) 
            {
                for (int k = j + 1; k <= n; k++) 
                {
  
                    // If the vertices form a triangle
                    if (adj[i][j] == 1 && 
                        adj[j][k] == 1 && adj[k][i] == 1)
                        return 0;
  
                    // If no edges are present
                    if (!(adj[i][j] == 1 || 
                          adj[j][k] == 1 || adj[k][i] == 1))
                        edgesNeeded = Math.min(edgesNeeded, 3);
  
                    else 
                    {
  
                        // If only 1 edge is required
                        if ((adj[i][j] == 1 && adj[j][k] == 1) || 
                            (adj[j][k] == 1 && adj[k][i] == 1) || 
                            (adj[k][i] == 1 && adj[i][j] == 1)) 
                        {
                            edgesNeeded = 1;
                        }
  
                        // Two edges are required
                        else
                            edgesNeeded = Math.min(edgesNeeded, 2);
                    }
                }
            }
        }
        return edgesNeeded;
    }
  
    // Driver Code
    public static void main(String[] args) 
    {
  
        // Number of nodes
        int n = 3;
  
        // Storing the edges in a vector of pairs
        Vector> v = new Vector<>(Arrays.asList(new Pair<>(1, 2), 
                                                             new Pair<>(1, 3)));
  
        System.out.println(minEdges(v, n));
    }
}
  
// This code is contributed by
// sanjeev2552


Python3
# Python3 implementation of the approach 
  
# Function to return the minimum number 
# of edges that need to be added to 
# the given graph such that it 
# contains at least one triangle 
def minEdges(v, n) : 
  
    # adj is the adjacency matrix such that 
    # adj[i][j] = 1 when there is an 
    # edge between i and j 
    adj = dict.fromkeys(range(n + 1)); 
      
    # adj.resize(n + 1); 
    for i in range(n + 1) :
        adj[i] = [0] * (n + 1); 
  
    # As the graph is undirected 
    # so there will be an edge 
    # between (i, j) and (j, i) 
    for i in range(len(v)) :
        adj[v[i][0]][v[i][1]] = 1; 
        adj[v[i][1]][v[i][0]] = 1; 
  
    # To store the required 
    # count of edges 
    edgesNeeded = 3; 
  
    # For every possible vertex triplet 
    for i in range(1, n + 1) : 
        for j in range(i + 1, n + 1) :
            for k in range(j + 1, n + 1) :
  
                # If the vertices form a triangle 
                if (adj[i][j] and adj[j][k] and adj[k][i]) :
                    return 0; 
  
                # If no edges are present 
                if (not (adj[i][j] or adj[j][k] or adj[k][i])) :
                    edgesNeeded = min(edgesNeeded, 3); 
  
                else :
  
                    # If only 1 edge is required 
                    if ((adj[i][j] and adj[j][k])
                        or (adj[j][k] and adj[k][i]) 
                        or (adj[k][i] and adj[i][j])) : 
                        edgesNeeded = 1; 
  
                    # Two edges are required 
                    else :
                        edgesNeeded = min(edgesNeeded, 2); 
      
    return edgesNeeded; 
  
# Driver code 
if __name__ == "__main__" : 
  
    # Number of nodes 
    n = 3; 
  
    # Storing the edges in a vector of pairs 
    v = [ [ 1, 2 ], [ 1, 3 ] ]; 
  
    print(minEdges(v, n)); 
      
# This code is contributed by kanugargng


C#
// C# implementation of the approach
using System;
using System.Collections.Generic;
  
class Pair 
{
    public int first;
    public int second;
  
    public Pair(int first, int second) 
    {
        this.first = first;
        this.second = second;
    }
}
  
class GFG 
{
  
    // Function to return the minimum number
    // of edges that need to be added to
    // the given graph such that it
    // contains at least one triangle
    static int minEdges(List v, int n) 
    {
  
        // adj is the adjacency matrix such that
        // adj[i,j] = 1 when there is an
        // edge between i and j
        int[,] adj = new int[n + 1, n + 1];
  
        // As the graph is undirected
        // so there will be an edge
        // between (i, j) and (j, i)
        for (int i = 0; i < v.Count; i++) 
        {
            adj[v[i].first,v[i].second] = 1;
            adj[v[i].second,v[i].first] = 1;
        }
  
        // To store the required
        // count of edges
        int edgesNeeded = 0;
  
        // For every possible vertex triplet
        for (int i = 1; i <= n; i++)
        {
            for (int j = i + 1; j <= n; j++) 
            {
                for (int k = j + 1; k <= n; k++) 
                {
  
                    // If the vertices form a triangle
                    if (adj[i, j] == 1 && 
                        adj[j, k] == 1 && adj[k, i] == 1)
                        return 0;
  
                    // If no edges are present
                    if (!(adj[i, j] == 1 || 
                        adj[j, k] == 1 || adj[k, i] == 1))
                        edgesNeeded = Math.Min(edgesNeeded, 3);
  
                    else
                    {
  
                        // If only 1 edge is required
                        if ((adj[i, j] == 1 && adj[j, k] == 1) || 
                            (adj[j, k] == 1 && adj[k, i] == 1) || 
                            (adj[k, i] == 1 && adj[i, j] == 1)) 
                        {
                            edgesNeeded = 1;
                        }
  
                        // Two edges are required
                        else
                            edgesNeeded = Math.Min(edgesNeeded, 2);
                    }
                }
            }
        }
        return edgesNeeded;
    }
  
    // Driver Code
    public static void Main(String[] args) 
    {
  
        // Number of nodes
        int n = 3;
  
        // Storing the edges in a vector of pairs
          
        List v = new List();
        v.Add(new Pair(1, 2));
        v.Add(new Pair(1, 3));
        Console.WriteLine(minEdges(v, n));
    }
}
  
// This code is contributed by PrinciRaj1992


输出:
1

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