C/C++，图算法——Dinic最大流量算法

1 文本格式

// C++ implementation of Dinic's Algorithm
#include<bits/stdc++.h>
using namespace std;

// A structure to represent a edge between
// two vertex
struct Edge
{
   int v; // Vertex v (or "to" vertex)
   // of a directed edge u-v. "From"
   // vertex u can be obtained using
   // index in adjacent array.

int flow; // flow of data in edge

int C; // capacity

   int rev; // To store index of reverse
   // edge in adjacency list so that
   // we can quickly find it.
};

// Residual Graph
class Graph
{
   int V; // number of vertex
   int* level; // stores level of a node
   vector< Edge >* adj;
public:
   Graph(int V)
   {
       adj = new vector<Edge>[V];
       this->V = V;
       level = new int[V];
   }

   // add edge to the graph
   void addEdge(int u, int v, int C)
   {
       // Forward edge : 0 flow and C capacity
       Edge a{ v, 0, C, adj[v].size() };

// Back edge : 0 flow and 0 capacity
Edge b{ u, 0, 0, adj[u].size() };

       adj[u].push_back(a);
       adj[v].push_back(b); // reverse edge
   }

   bool BFS(int s, int t);
   int sendFlow(int s, int flow, int t, int ptr[]);
   int DinicMaxflow(int s, int t);
};

// Finds if more flow can be sent from s to t.
// Also assigns levels to nodes.
bool Graph::BFS(int s, int t)
{
for (int i = 0; i < V; i++)
level[i] = -1;

level[s] = 0; // Level of source vertex

   // Create a queue, enqueue source vertex
   // and mark source vertex as visited here
   // level[] array works as visited array also.
   list< int > q;
   q.push_back(s);

   vector<Edge>::iterator i;
   while (!q.empty())
   {
       int u = q.front();
       q.pop_front();
       for (i = adj[u].begin(); i != adj[u].end(); i++)
       {
           Edge& e = *i;
           if (level[e.v] < 0 && e.flow < e.C)
           {
               // Level of current vertex is,
               // level of parent + 1
               level[e.v] = level[u] + 1;

               q.push_back(e.v);
           }
       }
   }

   // IF we can not reach to the sink we
   // return false else true
   return level[t] < 0 ? false : true;
}

// A DFS based function to send flow after BFS has
// figured out that there is a possible flow and
// constructed levels. This function called multiple
// times for a single call of BFS.
// flow : Current flow send by parent function call
// start[] : To keep track of next edge to be explored.
// start[i] stores count of edges explored
// from i.
// u : Current vertex
// t : Sink
int Graph::sendFlow(int u, int flow, int t, int start[])
{
   // Sink reached
   if (u == t)
       return flow;

   // Traverse all adjacent edges one -by - one.
   for (; start[u] < adj[u].size(); start[u]++)
   {
       // Pick next edge from adjacency list of u
       Edge& e = adj[u][start[u]];

       if (level[e.v] == level[u] + 1 && e.flow < e.C)
       {
           // find minimum flow from u to t
           int curr_flow = min(flow, e.C - e.flow);

int temp_flow = sendFlow(e.v, curr_flow, t, start);

           // flow is greater than zero
           if (temp_flow > 0)
           {
               // add flow to current edge
               e.flow += temp_flow;

               // subtract flow from reverse edge
               // of current edge
               adj[e.v][e.rev].flow -= temp_flow;
               return temp_flow;
           }
       }
   }

return 0;
}

// Returns maximum flow in graph
int Graph::DinicMaxflow(int s, int t)
{
   // Corner case
   if (s == t)
       return -1;

int total = 0; // Initialize result

   // Augment the flow while there is path
   // from source to sink
   while (BFS(s, t) == true)
   {
       // store how many edges are visited
       // from V { 0 to V }
       int* start = new int[V + 1];

// while flow is not zero in graph from S to D
while (int flow = sendFlow(s, INT_MAX, t, start))

           // Add path flow to overall flow
           total += flow;
   }

// return maximum flow
return total;
}

// Driver program to test above functions
int main()
{
   Graph g(6);
   g.addEdge(0, 1, 16);
   g.addEdge(0, 2, 13);
   g.addEdge(1, 2, 10);
   g.addEdge(1, 3, 12);
   g.addEdge(2, 1, 4);
   g.addEdge(2, 4, 14);
   g.addEdge(3, 2, 9);
   g.addEdge(3, 5, 20);
   g.addEdge(4, 3, 7);
   g.addEdge(4, 5, 4);

   // next exmp
   /*g.addEdge(0, 1, 3 );
   g.addEdge(0, 2, 7 ) ;
   g.addEdge(1, 3, 9);
   g.addEdge(1, 4, 9 );
   g.addEdge(2, 1, 9 );
   g.addEdge(2, 4, 9);
   g.addEdge(2, 5, 4);
   g.addEdge(3, 5, 3);
   g.addEdge(4, 5, 7 );
   g.addEdge(0, 4, 10);

   // next exp
   g.addEdge(0, 1, 10);
   g.addEdge(0, 2, 10);
   g.addEdge(1, 3, 4 );
   g.addEdge(1, 4, 8 );
   g.addEdge(1, 2, 2 );
   g.addEdge(2, 4, 9 );
   g.addEdge(3, 5, 10 );
   g.addEdge(4, 3, 6 );
   g.addEdge(4, 5, 10 ); */

cout << "Maximum flow " << g.DinicMaxflow(0, 5);
return 0;
}

2 代码格式

// C++ implementation of Dinic's Algorithm 
#include<bits/stdc++.h> 
using namespace std;

// A structure to represent a edge between 
// two vertex 
struct Edge
{
	int v;  // Vertex v (or "to" vertex) 
	// of a directed edge u-v. "From" 
	// vertex u can be obtained using 
	// index in adjacent array. 

	int flow; // flow of data in edge 

	int C;    // capacity 

	int rev; // To store index of reverse 
	// edge in adjacency list so that 
	// we can quickly find it. 
};

// Residual Graph 
class Graph
{
	int V; // number of vertex 
	int* level; // stores level of a node 
	vector< Edge >* adj;
public:
	Graph(int V)
	{
		adj = new vector<Edge>[V];
		this->V = V;
		level = new int[V];
	}

	// add edge to the graph 
	void addEdge(int u, int v, int C)
	{
		// Forward edge : 0 flow and C capacity 
		Edge a{ v, 0, C, adj[v].size() };

		// Back edge : 0 flow and 0 capacity 
		Edge b{ u, 0, 0, adj[u].size() };

		adj[u].push_back(a);
		adj[v].push_back(b); // reverse edge 
	}

	bool BFS(int s, int t);
	int sendFlow(int s, int flow, int t, int ptr[]);
	int DinicMaxflow(int s, int t);
};

// Finds if more flow can be sent from s to t. 
// Also assigns levels to nodes. 
bool Graph::BFS(int s, int t)
{
	for (int i = 0; i < V; i++)
		level[i] = -1;

	level[s] = 0;  // Level of source vertex 

	// Create a queue, enqueue source vertex 
	// and mark source vertex as visited here 
	// level[] array works as visited array also. 
	list< int > q;
	q.push_back(s);

	vector<Edge>::iterator i;
	while (!q.empty())
	{
		int u = q.front();
		q.pop_front();
		for (i = adj[u].begin(); i != adj[u].end(); i++)
		{
			Edge& e = *i;
			if (level[e.v] < 0 && e.flow < e.C)
			{
				// Level of current vertex is, 
				// level of parent + 1 
				level[e.v] = level[u] + 1;

				q.push_back(e.v);
			}
		}
	}

	// IF we can not reach to the sink we 
	// return false else true 
	return level[t] < 0 ? false : true;
}

// A DFS based function to send flow after BFS has 
// figured out that there is a possible flow and 
// constructed levels. This function called multiple 
// times for a single call of BFS. 
// flow : Current flow send by parent function call 
// start[] : To keep track of next edge to be explored. 
//           start[i] stores  count of edges explored 
//           from i. 
//  u : Current vertex 
//  t : Sink 
int Graph::sendFlow(int u, int flow, int t, int start[])
{
	// Sink reached 
	if (u == t)
		return flow;

	// Traverse all adjacent edges one -by - one. 
	for (; start[u] < adj[u].size(); start[u]++)
	{
		// Pick next edge from adjacency list of u 
		Edge& e = adj[u][start[u]];

		if (level[e.v] == level[u] + 1 && e.flow < e.C)
		{
			// find minimum flow from u to t 
			int curr_flow = min(flow, e.C - e.flow);

			int temp_flow = sendFlow(e.v, curr_flow, t, start);

			// flow is greater than zero 
			if (temp_flow > 0)
			{
				// add flow  to current edge 
				e.flow += temp_flow;

				// subtract flow from reverse edge 
				// of current edge 
				adj[e.v][e.rev].flow -= temp_flow;
				return temp_flow;
			}
		}
	}

	return 0;
}

// Returns maximum flow in graph 
int Graph::DinicMaxflow(int s, int t)
{
	// Corner case 
	if (s == t)
		return -1;

	int total = 0;  // Initialize result 

	// Augment the flow while there is path 
	// from source to sink 
	while (BFS(s, t) == true)
	{
		// store how many edges are visited 
		// from V { 0 to V } 
		int* start = new int[V + 1];

		// while flow is not zero in graph from S to D 
		while (int flow = sendFlow(s, INT_MAX, t, start))

			// Add path flow to overall flow 
			total += flow;
	}

	// return maximum flow 
	return total;
}

// Driver program to test above functions 
int main()
{
	Graph g(6);
	g.addEdge(0, 1, 16);
	g.addEdge(0, 2, 13);
	g.addEdge(1, 2, 10);
	g.addEdge(1, 3, 12);
	g.addEdge(2, 1, 4);
	g.addEdge(2, 4, 14);
	g.addEdge(3, 2, 9);
	g.addEdge(3, 5, 20);
	g.addEdge(4, 3, 7);
	g.addEdge(4, 5, 4);

	// next exmp 
	/*g.addEdge(0, 1, 3 );
	  g.addEdge(0, 2, 7 ) ;
	  g.addEdge(1, 3, 9);
	  g.addEdge(1, 4, 9 );
	  g.addEdge(2, 1, 9 );
	  g.addEdge(2, 4, 9);
	  g.addEdge(2, 5, 4);
	  g.addEdge(3, 5, 3);
	  g.addEdge(4, 5, 7 );
	  g.addEdge(0, 4, 10);

	 // next exp
	 g.addEdge(0, 1, 10);
	 g.addEdge(0, 2, 10);
	 g.addEdge(1, 3, 4 );
	 g.addEdge(1, 4, 8 );
	 g.addEdge(1, 2, 2 );
	 g.addEdge(2, 4, 9 );
	 g.addEdge(3, 5, 10 );
	 g.addEdge(4, 3, 6 );
	 g.addEdge(4, 5, 10 ); */

	cout << "Maximum flow " << g.DinicMaxflow(0, 5);
	return 0;
}

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