graph::HKGraph Class Reference

Represents Bipartite graph for Hopcroft Karp implementation. More...

Collaboration diagram for graph::HKGraph:

Public Member Functions
	HKGraph ()
	Default Constructor for initialization.
	HKGraph (int m, int n)
	Constructor for initialization. More...
void	addEdge (int u, int v)
	function to add edge from u to v More...
bool	bfs ()
	This function checks for the possibility of augmented path availability. More...
bool	dfs (int u)
	This functions checks whether an augmenting path is available exists beginning with free vertex u. More...
int	hopcroftKarpAlgorithm ()
	This function counts the number of augmenting paths between left and right sides of the Bipartite graph. More...

Private Attributes
int	m {}
	m is the number of vertices on left side of Bipartite Graph
int	n {}
	n is the number of vertices on right side of Bipartite Graph
const int	NIL {0}
const int	INF {INT_MAX}
std::vector< std::list< int > >	adj
	adj[u] stores adjacents of left side and 0 is used for dummy vertex
std::vector< int >	pair_u
	value of vertex 'u' ranges from 1 to m
std::vector< int >	pair_v
	value of vertex 'v' ranges from 1 to n
std::vector< int >	dist
	dist represents the distance between vertex 'u' and vertex 'v'

Detailed Description

Represents Bipartite graph for Hopcroft Karp implementation.

Constructor & Destructor Documentation

HKGraph()

graph::HKGraph::HKGraph	(	int	m,
		int	n
	)

Constructor for initialization.

Parameters

m	is the number of vertices on left side of Bipartite Graph
n	is the number of vertices on right side of Bipartite Graph

                             {
    this->m = m;
    this->n = n;
    adj = std::vector<std::list<int> >(m + 1);
}

Member Function Documentation

addEdge()

void graph::HKGraph::addEdge	(	int	u,
		int	v
	)

function to add edge from u to v

Parameters

u	is the position of first vertex
v	is the position of second vertex

{
    adj[u].push_back(v); // Add v to u’s list.
}

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bfs()

bool graph::HKGraph::bfs

(

)

This function checks for the possibility of augmented path availability.

Returns

true if there is an augmenting path available

false if there is no augmenting path available

{
    std::queue<int> q; // an integer queue for bfs
 
    // First layer of vertices (set distance as 0)
    for (int u = 1; u <= m; u++)
    {
        // If this is a free vertex, add it to queue
        if (pair_u[u] == NIL){
            
            dist[u] = 0; // u is not matched so distance is 0
            q.push(u);
        }
 
        else{
            dist[u] = INF; // set distance as infinite so that this vertex is considered next time for availibility
    }
    }
 
    
    dist[NIL] = INF; // Initialize distance to NIL as infinite
 
    // q is going to contain vertices of left side only.
    while (!q.empty())
    {
        int u = q.front();  // dequeue a vertex
        q.pop();
 
        // If this node is not NIL and can provide a shorter path to NIL then
        if (dist[u] < dist[NIL])
        {
            // Get all the adjacent vertices of the dequeued vertex u
            std::list<int>::iterator it;
            for (it = adj[u].begin(); it != adj[u].end(); ++it)
            {
                int v = *it;
 
                // If pair of v is not considered so far i.e. (v, pair_v[v]) is not yet explored edge.
                if (dist[pair_v[v]] == INF)
                {
                    dist[pair_v[v]] = dist[u] + 1; 
                    q.push(pair_v[v]);    // Consider the pair and push it to queue
                }
            }
        }
    }
 
   
   
    return (dist[NIL] != INF);   // If we could come back to NIL using alternating path of distinct vertices then there is an augmenting path available
}

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dfs()

bool graph::HKGraph::dfs

(

int

u

)

This functions checks whether an augmenting path is available exists beginning with free vertex u.

Parameters

u	represents position of vertex

Returns

true if there is an augmenting path beginning with free vertex u

false if there is no augmenting path beginning with free vertex u

{
    if (u != NIL)
    {
        std::list<int>::iterator it;
        for (it = adj[u].begin(); it != adj[u].end(); ++it)
        {
            
            int v = *it; // Adjacent vertex of u
 
            // Follow the distances set by BFS search
            if (dist[pair_v[v]] == dist[u] + 1)
            {
                // If dfs for pair of v also return true then new matching possible, store the matching
                if (dfs(pair_v[v]) == true)
                {   
                    pair_v[v] = u;
                    pair_u[u] = v;
                    return true;
                }
            }
        }
 
        
        dist[u] = INF; // If there is no augmenting path beginning with u then set distance to infinite.
        return false;
    }
    return true;
}

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hopcroftKarpAlgorithm()

int graph::HKGraph::hopcroftKarpAlgorithm

(

)

This function counts the number of augmenting paths between left and right sides of the Bipartite graph.

Returns

size of maximum matching

{
 
    // pair_u[u] stores pair of u in matching on left side of Bipartite Graph.
    // If u doesn't have any pair, then pair_u[u] is NIL
    pair_u = std::vector<int>(m + 1,NIL); 
 
    // pair_v[v] stores pair of v in matching on right side of Biparite Graph.
    // If v doesn't have any pair, then pair_u[v] is NIL
    pair_v = std::vector<int>(n + 1,NIL); 
 
    dist = std::vector<int>(m + 1);  // dist[u] stores distance of left side vertices
 
    int result = 0;  // Initialize result
 
    // Keep updating the result while there is an augmenting path possible.
    while (bfs())
    {
        // Find a free vertex to check for a matching
        for (int u = 1; u <= m; u++){
 
            // If current vertex is free and there is
            // an augmenting path from current vertex
            // then increment the result
            if (pair_u[u] == NIL && dfs(u)){
                result++;
        }
    }
    }
    return result;
}

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The documentation for this class was generated from the following file:

• graph/hopcroft_karp.cpp