Karp's minimum mean (or average) weight cycle algorithm

Karp’s minimum mean (or average) weight cycle algorithm

Difficulty Level : Expert
Last Updated : 29 Oct, 2020

Given a directed and strongly connected graph with non-negative edge weights. We define the mean weight of a cycle as the summation of all the edge weights of the cycle divided by the no. of edges. Our task is to find the minimum mean weight among all the directed cycles of the graph.
Example:

Input : Below Graph

karps_mean_value

Output : 1.66667

Method to find the smallest mean weight value cycle efficiently

Step 1: Choose first vertex as source.

Step 2: Compute the shortest path to all other vertices 
        on a path consisting of k edges 0 <= k <= V 
        where V is number of vertices.
        This is a simple dp problem which can be computed 
        by the recursive solution
        dp[k][v] = min(dp[k][v], dp[k-1][u] + weight(u,v)
        where v is the destination and the edge(u,v) should
        belong to E

Step 3: For each vertex calculate max(dp[n][v]-dp[k][v])/(n-k) 
         where 0<=k<=n-1

Step 4: The minimum of the values calculated above is the 
        required answer.

Please refer to the solution of problem 9.2 here for proof that the above steps find minimum average weight.

C++

// C++ program to find minimum average
// weight of a cycle in connected and
// directed graph.
#include<bits/stdc++.h>
using namespace std;
 
const int V = 4;
 
// a struct to represent edges
struct edge
{
    int from, weight;
};
 
// vector to store edges
vector <edge> edges[V];
 
void addedge(int u,int v,int w)
{
    edges[v].push_back({u, w});
}
 
// calculates the shortest path
void shortestpath(int dp[][V])
{
    // initializing all distances as -1
    for (int i=0; i<=V; i++)
        for (int j=0; j<V; j++)
            dp[i][j] = -1;
 
    // shortest distance from first vertex
    // to in tself consisting of 0 edges
    dp[0][0] = 0;
 
    // filling up the dp table
    for (int i=1; i<=V; i++)
    {
        for (int j=0; j<V; j++)
        {
            for (int k=0; k<edges[j].size(); k++)
            {
                if (dp[i-1][edges[j][k].from] != -1)
                {
                    int curr_wt = dp[i-1][edges[j][k].from] +
                                  edges[j][k].weight;
                    if (dp[i][j] == -1)
                        dp[i][j] = curr_wt;
                    else
                       dp[i][j] = min(dp[i][j], curr_wt);
                }
            }
        }
    }
}
 
// Returns minimum value of average weight of a
// cycle in graph.
double minAvgWeight()
{
    int dp[V+1][V];
    shortestpath(dp);
 
    // array to store the avg values
    double avg[V];
    for (int i=0; i<V; i++)
        avg[i] = -1;
 
    // Compute average values for all vertices using
    // weights of shortest paths store in dp.
    for (int i=0; i<V; i++)
    {
        if (dp[V][i] != -1)
        {
            for (int j=0; j<V; j++)
                if (dp[j][i] != -1)
                    avg[i] = max(avg[i],
                ((double)dp[V][i]-dp[j][i])/(V-j));
        }
    }
 
    // Find minimum value in avg[]
    double result = avg[0];
    for (int i=0; i<V; i++)
        if (avg[i] != -1 && avg[i] < result)
            result = avg[i];
 
    return result;
}
 
// Driver function
int main()
{
    addedge(0, 1, 1);
    addedge(0, 2, 10);
    addedge(1, 2, 3);
    addedge(2, 3, 2);
    addedge(3, 1, 0);
    addedge(3, 0, 8);
 
    cout << minAvgWeight();
 
    return 0;
}

Java

// Java program to find minimum average 
// weight of a cycle in connected and 
// directed graph.
import java.io.*;
import java.util.*;
 
class GFG 
{
static int V = 4;
 
// a struct to represent edges
static class Edge
{
    int from, weight;
 
    Edge(int from, int weight)
    {
        this.from = from;
        this.weight = weight;
    }
}
 
// vector to store edges
//@SuppressWarnings("unchecked")
static Vector<Edge>[] edges = new Vector[V];
static
{
    for (int i = 0; i < V; i++)
        edges[i] = new Vector<>();
}
 
static void addedge(int u, int v, int w) 
{
    edges[v].add(new Edge(u, w));
}
 
// calculates the shortest path
static void shortestpath(int[][] dp) 
{
    // initializing all distances as -1
    for (int i = 0; i <= V; i++)
        for (int j = 0; j < V; j++)
            dp[i][j] = -1;
 
    // shortest distance from first vertex
    // to in tself consisting of 0 edges
    dp[0][0] = 0;
 
    // filling up the dp table
    for (int i = 1; i <= V; i++)
    {
        for (int j = 0; j < V; j++) 
        {
            for (int k = 0; k < edges[j].size(); k++)
            {
                if (dp[i - 1][edges[j].elementAt(k).from] != -1) 
                {
                    int curr_wt = dp[i - 1][edges[j].elementAt(k).from] + 
                                            edges[j].elementAt(k).weight;
                    if (dp[i][j] == -1)
                        dp[i][j] = curr_wt;
                    else
                        dp[i][j] = Math.min(dp[i][j], curr_wt);
                }
            }
        }
    }
}
 
// Returns minimum value of average weight 
// of a cycle in graph.
static double minAvgWeight() 
{
    int[][] dp = new int[V + 1][V];
    shortestpath(dp);
 
    // array to store the avg values
    double[] avg = new double[V];
    for (int i = 0; i < V; i++)
        avg[i] = -1;
 
    // Compute average values for all vertices using
    // weights of shortest paths store in dp.
    for (int i = 0; i < V; i++) 
    {
        if (dp[V][i] != -1) 
        {
            for (int j = 0; j < V; j++)
                if (dp[j][i] != -1)
                    avg[i] = Math.max(avg[i], 
                                    ((double) dp[V][i] - 
                                              dp[j][i]) / (V - j));
        }
    }
 
    // Find minimum value in avg[]
    double result = avg[0];
    for (int i = 0; i < V; i++)
        if (avg[i] != -1 && avg[i] < result)
            result = avg[i];
 
    return result;
}
 
// Driver Code
public static void main(String[] args) 
{
    addedge(0, 1, 1);
    addedge(0, 2, 10);
    addedge(1, 2, 3);
    addedge(2, 3, 2);
    addedge(3, 1, 0);
    addedge(3, 0, 8);
 
    System.out.printf("%.5f", minAvgWeight());
}
}
 
// This code is contributed by
// sanjeev2552

Python3

# Python3 program to find minimum 
# average weight of a cycle in 
# connected and directed graph. 
 
# a struct to represent edges 
class edge:
    def __init__(self, u, w):
        self.From = u
        self.weight = w
 
def addedge(u, v, w):
    edges[v].append(edge(u, w))
 
# calculates the shortest path 
def shortestpath(dp):
     
    # initializing all distances as -1
    for i in range(V + 1):
        for j in range(V):
            dp[i][j] = -1
 
    # shortest distance From first vertex 
    # to in tself consisting of 0 edges 
    dp[0][0] = 0
 
    # filling up the dp table
    for i in range(1, V + 1):
        for j in range(V):
            for k in range(len(edges[j])):
                if (dp[i - 1][edges[j][k].From] != -1):
                    curr_wt = (dp[i - 1][edges[j][k].From] +
                                         edges[j][k].weight) 
                    if (dp[i][j] == -1): 
                        dp[i][j] = curr_wt 
                    else:
                        dp[i][j] = min(dp[i][j], curr_wt)
 
# Returns minimum value of average 
# weight of a cycle in graph. 
def minAvgWeight():
    dp = [[None] * V for i in range(V + 1)]
    shortestpath(dp) 
 
    # array to store the avg values 
    avg = [-1] * V
 
    # Compute average values for all 
    # vertices using weights of 
    # shortest paths store in dp.
    for i in range(V):
        if (dp[V][i] != -1):
            for j in range(V):
                if (dp[j][i] != -1): 
                    avg[i] = max(avg[i], (dp[V][i] -
                                          dp[j][i]) / (V - j))
 
    # Find minimum value in avg[] 
    result = avg[0]
    for i in range(V):
        if (avg[i] != -1 and avg[i] < result): 
            result = avg[i] 
 
    return result
 
# Driver Code
V = 4
 
# vector to store edges 
edges = [[] for i in range(V)]
 
addedge(0, 1, 1) 
addedge(0, 2, 10) 
addedge(1, 2, 3) 
addedge(2, 3, 2) 
addedge(3, 1, 0) 
addedge(3, 0, 8) 
 
print(minAvgWeight())
 
# This code is contributed by Pranchalk

C#

// C# program to find minimum 
// average weight of a cycle 
// in connected and directed graph. 
using System;
using System.Collections.Generic;
class GFG{ 
   
static int V = 4; 
 
// a struct to represent
// edges 
public class Edge 
{ 
  public int from, weight; 
  public Edge(int from, 
              int weight) 
  { 
    this.from = from; 
    this.weight = weight; 
  } 
} 
 
// vector to store edges  
static List<Edge>[] edges = 
            new List<Edge>[V];  
 
static void addedge(int u, 
                    int v, int w) 
{ 
  edges[v].Add(new Edge(u, w)); 
} 
 
// calculates the shortest path 
static void shortestpath(int[,] dp) 
{ 
  // initializing all distances 
  // as -1 
  for (int i = 0; i <= V; i++) 
    for (int j = 0; j < V; j++) 
      dp[i, j] = -1; 
 
  // shortest distance from 
  // first vertex to in tself 
  // consisting of 0 edges 
  dp[0, 0] = 0; 
 
  // filling up the dp table 
  for (int i = 1; i <= V; i++) 
  { 
    for (int j = 0; j < V; j++) 
    { 
      for (int k = 0; 
               k < edges[j].Count; k++) 
      { 
        if (dp[i - 1, 
               edges[j][k].from] != -1) 
        { 
          int curr_wt = dp[i - 1,
                           edges[j][k].from] + 
                           edges[j][k].weight; 
          if (dp[i, j] == -1) 
            dp[i, j] = curr_wt; 
          else
            dp[i, j] = Math.Min(dp[i, j], 
                                curr_wt); 
        } 
      } 
    } 
  } 
} 
 
// Returns minimum value of 
// average weight of a cycle 
// in graph. 
static double minAvgWeight() 
{ 
  int[,] dp = new int[V + 1, V]; 
  shortestpath(dp); 
 
  // array to store the 
  // avg values 
  double[] avg = new double[V]; 
   
  for (int i = 0; i < V; i++) 
    avg[i] = -1; 
 
  // Compute average values for 
  // all vertices using weights 
  // of shortest paths store in dp. 
  for (int i = 0; i < V; i++) 
  { 
    if (dp[V, i] != -1) 
    { 
      for (int j = 0; j < V; j++) 
        if (dp[j, i] != -1) 
          avg[i] = Math.Max(avg[i], 
                           ((double) dp[V, i] - 
                             dp[j, i]) / 
                             (V - j)); 
    } 
  } 
 
  // Find minimum value in avg[] 
  double result = avg[0]; 
   
  for (int i = 0; i < V; i++) 
    if (avg[i] != -1 && 
        avg[i] < result) 
      result = avg[i]; 
 
  return result; 
} 
 
// Driver Code 
public static void Main(String[] args) 
{ 
  for (int i = 0; i < V; i++) 
    edges[i] = new List<Edge>();
   
  addedge(0, 1, 1); 
  addedge(0, 2, 10); 
  addedge(1, 2, 3); 
  addedge(2, 3, 2); 
  addedge(3, 1, 0); 
  addedge(3, 0, 8); 
 
  Console.Write("{0:F5}",
                minAvgWeight()); 
} 
} 
 
// This code is contributed by Princi Singh

Output:

1.66667

Here the graph with no cycle will return value as -1.
Reference:
https://courses.csail.mit.edu/6.046/fall01/handouts/ps9sol.pdf
https://www.hackerearth.com/practice/notes/karp-minimum-mean-weighted-cycle/
Introduction to Algorithms Third Edition page 681 by Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest and Clifford Stein
This article is contributed by Ayush Jha. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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