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Árvore geradora de produto mínimo

Dado um grafo conectado e não direcionado, uma árvore geradora desse grafo é um subgrafo que é uma árvore e conecta todos os vértices. Um único gráfico pode ter muitas árvores geradoras diferentes. Uma árvore geradora de produto mínimo para um gráfico conectado e não direcionado ponderado é uma árvore geradora com um produto de peso menor ou igual ao produto de peso de qualquer outra árvore geradora. O produto dos pesos de uma árvore geradora é o produto dos pesos correspondentes a cada aresta da árvore geradora. Todos os pesos do gráfico fornecido serão positivos por simplicidade.

Exemplos:

Árvore geradora de produto mínimo



Minimum Product that we can obtain is 180 for above graph by choosing edges 0-1 1-2 0-3 and 1-4

Este problema pode ser resolvido usando algoritmos padrão de árvore geradora mínima como Kruskal ( https://www.geeksforgeeks.org/dsa/kruskals-minimum-spanning-tree-algorithm-greedy-algo-2/ )e afetado algoritmo de , mas precisamos modificar nosso gráfico para usar esses algoritmos. Algoritmos de árvore geradora mínima tentam minimizar a soma total dos pesos. Aqui precisamos minimizar o produto total dos pesos. Podemos usar a propriedade de logaritmos para superar esse problema. 
Como sabemos 

 log(w1* w2 * w3 * …. * wN) = log(w1) + log(w2) + log(w3) ….. + log(wN)


Podemos substituir cada peso do gráfico por seu valor de log e então aplicar qualquer algoritmo de árvore geradora mínima que tentará minimizar a soma de log (wi) que por sua vez minimiza o produto de peso. 

 

Árvore geradora de produto mínimo


No código abaixo, primeiro construímos o gráfico de log a partir do gráfico de entrada fornecido e, em seguida, esse gráfico é fornecido como entrada para o algoritmo MST do prim, que minimizará a soma total dos pesos da árvore. Como os pesos do gráfico modificado são logaritmos do gráfico de entrada real, na verdade minimizamos o produto dos pesos da árvore geradora.  

C++
// A C++ program for getting minimum product // spanning tree The program is for adjacency matrix // representation of the graph #include    // Number of vertices in the graph #define V 5 // A utility function to find the vertex with minimum // key value from the set of vertices not yet included // in MST int minKey(int key[] bool mstSet[]) {  // Initialize min value  int min = INT_MAX min_index;  for (int v = 0; v < V; v++)  if (mstSet[v] == false && key[v] < min)  min = key[v] min_index = v;  return min_index; } // A utility function to print the constructed MST // stored in parent[] and print Minimum Obtainable // product int printMST(int parent[] int n int graph[V][V]) {  printf('Edge Weightn');  int minProduct = 1;  for (int i = 1; i < V; i++) {  printf('%d - %d %d n'  parent[i] i graph[i][parent[i]]);  minProduct *= graph[i][parent[i]];  }  printf('Minimum Obtainable product is %dn'  minProduct); } // Function to construct and print MST for a graph // represented using adjacency matrix representation // inputGraph is sent for printing actual edges and // logGraph is sent for actual MST operations void primMST(int inputGraph[V][V] double logGraph[V][V]) {  int parent[V]; // Array to store constructed MST  int key[V]; // Key values used to pick minimum  // weight edge in cut  bool mstSet[V]; // To represent set of vertices not  // yet included in MST  // Initialize all keys as INFINITE  for (int i = 0; i < V; i++)  key[i] = INT_MAX mstSet[i] = false;  // Always include first 1st vertex in MST.  key[0] = 0; // Make key 0 so that this vertex is  // picked as first vertex  parent[0] = -1; // First node is always root of MST  // The MST will have V vertices  for (int count = 0; count < V - 1; count++) {  // Pick the minimum key vertex from the set of  // vertices not yet included in MST  int u = minKey(key mstSet);  // Add the picked vertex to the MST Set  mstSet[u] = true;  // Update key value and parent index of the  // adjacent vertices of the picked vertex.  // Consider only those vertices which are not yet  // included in MST  for (int v = 0; v < V; v++)  // logGraph[u][v] is non zero only for  // adjacent vertices of m mstSet[v] is false  // for vertices not yet included in MST  // Update the key only if logGraph[u][v] is  // smaller than key[v]  if (logGraph[u][v] > 0 && mstSet[v] == false && logGraph[u][v] < key[v])  parent[v] = u key[v] = logGraph[u][v];  }  // print the constructed MST  printMST(parent V inputGraph); } // Method to get minimum product spanning tree void minimumProductMST(int graph[V][V]) {  double logGraph[V][V];  // Constructing logGraph from original graph  for (int i = 0; i < V; i++) {  for (int j = 0; j < V; j++) {  if (graph[i][j] > 0)  logGraph[i][j] = log(graph[i][j]);  else  logGraph[i][j] = 0;  }  }  // Applying standard Prim's MST algorithm on  // Log graph.  primMST(graph logGraph); } // driver program to test above function int main() {  /* Let us create the following graph  2 3  (0)--(1)--(2)  | /  |  6| 8/ 5 |7  | /  |  (3)-------(4)  9 */  int graph[V][V] = {  { 0 2 0 6 0 }  { 2 0 3 8 5 }  { 0 3 0 0 7 }  { 6 8 0 0 9 }  { 0 5 7 9 0 }  };  // Print the solution  minimumProductMST(graph);  return 0; } 
Java
// A Java program for getting minimum product // spanning tree The program is for adjacency matrix // representation of the graph import java.util.*; class GFG {  // Number of vertices in the graph  static int V = 5;  // A utility function to find the vertex with minimum  // key value from the set of vertices not yet included  // in MST  static int minKey(int key[] boolean[] mstSet)  {  // Initialize min value  int min = Integer.MAX_VALUE min_index = 0;  for (int v = 0; v < V; v++) {  if (mstSet[v] == false && key[v] < min) {  min = key[v];  min_index = v;  }  }  return min_index;  }  // A utility function to print the constructed MST  // stored in parent[] and print Minimum Obtainable  // product  static void printMST(int parent[] int n int graph[][])  {  System.out.printf('Edge Weightn');  int minProduct = 1;  for (int i = 1; i < V; i++) {  System.out.printf('%d - %d %d n'  parent[i] i graph[i][parent[i]]);  minProduct *= graph[i][parent[i]];  }  System.out.printf('Minimum Obtainable product is %dn'  minProduct);  }  // Function to construct and print MST for a graph  // represented using adjacency matrix representation  // inputGraph is sent for printing actual edges and  // logGraph is sent for actual MST operations  static void primMST(int inputGraph[][] double logGraph[][])  {  int[] parent = new int[V]; // Array to store constructed MST  int[] key = new int[V]; // Key values used to pick minimum  // weight edge in cut  boolean[] mstSet = new boolean[V]; // To represent set of vertices not  // yet included in MST  // Initialize all keys as INFINITE  for (int i = 0; i < V; i++) {  key[i] = Integer.MAX_VALUE;  mstSet[i] = false;  }  // Always include first 1st vertex in MST.  key[0] = 0; // Make key 0 so that this vertex is  // picked as first vertex  parent[0] = -1; // First node is always root of MST  // The MST will have V vertices  for (int count = 0; count < V - 1; count++) {  // Pick the minimum key vertex from the set of  // vertices not yet included in MST  int u = minKey(key mstSet);  // Add the picked vertex to the MST Set  mstSet[u] = true;  // Update key value and parent index of the  // adjacent vertices of the picked vertex.  // Consider only those vertices which are not yet  // included in MST  for (int v = 0; v < V; v++) // logGraph[u][v] is non zero only for  // adjacent vertices of m mstSet[v] is false  // for vertices not yet included in MST  // Update the key only if logGraph[u][v] is  // smaller than key[v]  {  if (logGraph[u][v] > 0  && mstSet[v] == false  && logGraph[u][v] < key[v]) {  parent[v] = u;  key[v] = (int)logGraph[u][v];  }  }  }  // print the constructed MST  printMST(parent V inputGraph);  }  // Method to get minimum product spanning tree  static void minimumProductMST(int graph[][])  {  double[][] logGraph = new double[V][V];  // Constructing logGraph from original graph  for (int i = 0; i < V; i++) {  for (int j = 0; j < V; j++) {  if (graph[i][j] > 0) {  logGraph[i][j] = Math.log(graph[i][j]);  }  else {  logGraph[i][j] = 0;  }  }  }  // Applying standard Prim's MST algorithm on  // Log graph.  primMST(graph logGraph);  }  // Driver code  public static void main(String[] args)  {  /* Let us create the following graph  2 3  (0)--(1)--(2)  | /  |  6| 8/ 5 |7  | /  |  (3)-------(4)  9 */  int graph[][] = {  { 0 2 0 6 0 }  { 2 0 3 8 5 }  { 0 3 0 0 7 }  { 6 8 0 0 9 }  { 0 5 7 9 0 }  };  // Print the solution  minimumProductMST(graph);  } } // This code has been contributed by 29AjayKumar 
Python3
# A Python3 program for getting minimum # product spanning tree The program is  # for adjacency matrix representation # of the graph  import math # Number of vertices in the graph  V = 5 # A utility function to find the vertex # with minimum key value from the set  # of vertices not yet included in MST  def minKey(key mstSet): # Initialize min value  min = 10000000 min_index = 0 for v in range(V): if (mstSet[v] == False and key[v] < min): min = key[v] min_index = v return min_index # A utility function to print the constructed  # MST stored in parent[] and print Minimum  # Obtainable product  def printMST(parent n graph): print('Edge Weight') minProduct = 1 for i in range(1 V): print('{} - {} {} '.format(parent[i] i graph[i][parent[i]])) minProduct *= graph[i][parent[i]] print('Minimum Obtainable product is {}'.format( minProduct)) # Function to construct and print MST for  # a graph represented using adjacency  # matrix representation inputGraph is # sent for printing actual edges and  # logGraph is sent for actual MST  # operations  def primMST(inputGraph logGraph): # Array to store constructed MST  parent = [0 for i in range(V)] # Key values used to pick minimum  key = [10000000 for i in range(V)] # weight edge in cut  # To represent set of vertices not  mstSet = [False for i in range(V)] # Yet included in MST  # Always include first 1st vertex in MST  # Make key 0 so that this vertex is  key[0] = 0 # Picked as first vertex  # First node is always root of MST  parent[0] = -1 # The MST will have V vertices  for count in range(0 V - 1): # Pick the minimum key vertex from # the set of vertices not yet  # included in MST  u = minKey(key mstSet) # Add the picked vertex to the MST Set  mstSet[u] = True # Update key value and parent index # of the adjacent vertices of the  # picked vertex. Consider only those  # vertices which are not yet  # included in MST  for v in range(V): # logGraph[u][v] is non zero only # for adjacent vertices of m  # mstSet[v] is false for vertices # not yet included in MST. Update  # the key only if logGraph[u][v] is  # smaller than key[v]  if (logGraph[u][v] > 0 and mstSet[v] == False and logGraph[u][v] < key[v]): parent[v] = u key[v] = logGraph[u][v] # Print the constructed MST  printMST(parent V inputGraph) # Method to get minimum product spanning tree  def minimumProductMST(graph): logGraph = [[0 for j in range(V)] for i in range(V)] # Constructing logGraph from  # original graph  for i in range(V): for j in range(V): if (graph[i][j] > 0): logGraph[i][j] = math.log(graph[i][j]) else: logGraph[i][j] = 0 # Applying standard Prim's MST algorithm # on Log graph.  primMST(graph logGraph) # Driver code if __name__=='__main__':    ''' Let us create the following graph   2 3   (0)--(1)--(2)   | /  |   6| 8/ 5 |7   | /  |   (3)-------(4)   9 ''' graph = [ [ 0 2 0 6 0 ] [ 2 0 3 8 5 ] [ 0 3 0 0 7 ] [ 6 8 0 0 9 ] [ 0 5 7 9 0 ] ] # Print the solution  minimumProductMST(graph) # This code is contributed by rutvik_56 
C#
// C# program for getting minimum product // spanning tree The program is for adjacency matrix // representation of the graph using System; class GFG {  // Number of vertices in the graph  static int V = 5;  // A utility function to find the vertex with minimum  // key value from the set of vertices not yet included  // in MST  static int minKey(int[] key Boolean[] mstSet)  {  // Initialize min value  int min = int.MaxValue min_index = 0;  for (int v = 0; v < V; v++) {  if (mstSet[v] == false && key[v] < min) {  min = key[v];  min_index = v;  }  }  return min_index;  }  // A utility function to print the constructed MST  // stored in parent[] and print Minimum Obtainable  // product  static void printMST(int[] parent int n int[ ] graph)  {  Console.Write('Edge Weightn');  int minProduct = 1;  for (int i = 1; i < V; i++) {  Console.Write('{0} - {1} {2} n'  parent[i] i graph[i parent[i]]);  minProduct *= graph[i parent[i]];  }  Console.Write('Minimum Obtainable product is {0}n'  minProduct);  }  // Function to construct and print MST for a graph  // represented using adjacency matrix representation  // inputGraph is sent for printing actual edges and  // logGraph is sent for actual MST operations  static void primMST(int[ ] inputGraph double[ ] logGraph)  {  int[] parent = new int[V]; // Array to store constructed MST  int[] key = new int[V]; // Key values used to pick minimum  // weight edge in cut  Boolean[] mstSet = new Boolean[V]; // To represent set of vertices not  // yet included in MST  // Initialize all keys as INFINITE  for (int i = 0; i < V; i++) {  key[i] = int.MaxValue;  mstSet[i] = false;  }  // Always include first 1st vertex in MST.  key[0] = 0; // Make key 0 so that this vertex is  // picked as first vertex  parent[0] = -1; // First node is always root of MST  // The MST will have V vertices  for (int count = 0; count < V - 1; count++) {  // Pick the minimum key vertex from the set of  // vertices not yet included in MST  int u = minKey(key mstSet);  // Add the picked vertex to the MST Set  mstSet[u] = true;  // Update key value and parent index of the  // adjacent vertices of the picked vertex.  // Consider only those vertices which are not yet  // included in MST  for (int v = 0; v < V; v++) // logGraph[u v] is non zero only for  // adjacent vertices of m mstSet[v] is false  // for vertices not yet included in MST  // Update the key only if logGraph[u v] is  // smaller than key[v]  {  if (logGraph[u v] > 0  && mstSet[v] == false  && logGraph[u v] < key[v]) {  parent[v] = u;  key[v] = (int)logGraph[u v];  }  }  }  // print the constructed MST  printMST(parent V inputGraph);  }  // Method to get minimum product spanning tree  static void minimumProductMST(int[ ] graph)  {  double[ ] logGraph = new double[V V];  // Constructing logGraph from original graph  for (int i = 0; i < V; i++) {  for (int j = 0; j < V; j++) {  if (graph[i j] > 0) {  logGraph[i j] = Math.Log(graph[i j]);  }  else {  logGraph[i j] = 0;  }  }  }  // Applying standard Prim's MST algorithm on  // Log graph.  primMST(graph logGraph);  }  // Driver code  public static void Main(String[] args)  {  /* Let us create the following graph  2 3  (0)--(1)--(2)  | /  |  6| 8/ 5 |7  | /  |  (3)-------(4)  9 */  int[ ] graph = {  { 0 2 0 6 0 }  { 2 0 3 8 5 }  { 0 3 0 0 7 }  { 6 8 0 0 9 }  { 0 5 7 9 0 }  };  // Print the solution  minimumProductMST(graph);  } } /* This code contributed by PrinciRaj1992 */ 
JavaScript
<script> // A Javascript program for getting minimum product // spanning tree The program is for adjacency matrix // representation of the graph // Number of vertices in the graph let V = 5; // A utility function to find the vertex with minimum  // key value from the set of vertices not yet included  // in MST function minKey(keymstSet) {  // Initialize min value  let min = Number.MAX_VALUE min_index = 0;    for (let v = 0; v < V; v++) {  if (mstSet[v] == false && key[v] < min) {  min = key[v];  min_index = v;  }  }  return min_index; } // A utility function to print the constructed MST  // stored in parent[] and print Minimum Obtainable  // product function printMST(parentngraph) {  document.write('Edge Weight  
'
); let minProduct = 1; for (let i = 1; i < V; i++) { document.write( parent[i]+' - '+ i+' ' +graph[i][parent[i]]+'
'
); minProduct *= graph[i][parent[i]]; } document.write('Minimum Obtainable product is ' minProduct+'
'
); } // Function to construct and print MST for a graph // represented using adjacency matrix representation // inputGraph is sent for printing actual edges and // logGraph is sent for actual MST operations function primMST(inputGraphlogGraph) { let parent = new Array(V); // Array to store constructed MST let key = new Array(V); // Key values used to pick minimum // weight edge in cut let mstSet = new Array(V); // To represent set of vertices not // yet included in MST // Initialize all keys as INFINITE for (let i = 0; i < V; i++) { key[i] = Number.MAX_VALUE; mstSet[i] = false; } // Always include first 1st vertex in MST. key[0] = 0; // Make key 0 so that this vertex is // picked as first vertex parent[0] = -1; // First node is always root of MST // The MST will have V vertices for (let count = 0; count < V - 1; count++) { // Pick the minimum key vertex from the set of // vertices not yet included in MST let u = minKey(key mstSet); // Add the picked vertex to the MST Set mstSet[u] = true; // Update key value and parent index of the // adjacent vertices of the picked vertex. // Consider only those vertices which are not yet // included in MST for (let v = 0; v < V; v++) // logGraph[u][v] is non zero only for // adjacent vertices of m mstSet[v] is false // for vertices not yet included in MST // Update the key only if logGraph[u][v] is // smaller than key[v] { if (logGraph[u][v] > 0 && mstSet[v] == false && logGraph[u][v] < key[v]) { parent[v] = u; key[v] = logGraph[u][v]; } } } // print the constructed MST printMST(parent V inputGraph); } // Method to get minimum product spanning tree function minimumProductMST(graph) { let logGraph = new Array(V); // Constructing logGraph from original graph for (let i = 0; i < V; i++) { logGraph[i]=new Array(V); for (let j = 0; j < V; j++) { if (graph[i][j] > 0) { logGraph[i][j] = Math.log(graph[i][j]); } else { logGraph[i][j] = 0; } } } // Applying standard Prim's MST algorithm on // Log graph. primMST(graph logGraph); } // Driver code /* Let us create the following graph 2 3 (0)--(1)--(2) | / | 6| 8/ 5 |7 | / | (3)-------(4) 9 */ let graph = [ [ 0 2 0 6 0 ] [ 2 0 3 8 5 ] [ 0 3 0 0 7 ] [ 6 8 0 0 9 ] [ 0 5 7 9 0 ] ]; // Print the solution minimumProductMST(graph); // This code is contributed by rag2127 </script>

Saída:  

Edge Weight 0 - 1 2 1 - 2 3 0 - 3 6 1 - 4 5 Minimum Obtainable product is 180

O complexidade de tempo deste algoritmo é O(V2), pois há dois loops for aninhados que iteram sobre todos os vértices. 

O complexidade do espaço deste algoritmo é O(V2), pois estamos usando uma matriz 2-D de tamanho V x V para armazenar o gráfico de entrada.

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