poj 1273 Drainage Ditches 最大流

        第一次写最大流，照着书上面敲。

        用Edmonds_karp算法，每次通过bfs搜出一条增广路径加到流中，直到不能搜到为止。这题要注意有重边，多条重边要加在一起，用+=

 

#include <iostream>

#include <queue>

using namespace std;



const int N = 205;

const int INF = 1000000000;



int c[N][N];

int flow[N][N];

int min_flow[N];

int pre[N];



int n, m;



int max_flow()

{

	memset(flow, 0, sizeof(flow));



	int u;

	int ans = 0;



	while (1)

	{

		memset(pre, -1, sizeof(pre));	

		memset(min_flow, 0, sizeof(min_flow));

		queue<int> Q;

		Q.push(1);



		min_flow[1] = INF;

		while (!Q.empty())

		{

			u = Q.front();

			Q.pop();



			if (u == m) 

				break;



			for (int i = 2; i <= m; i++)

				if (pre[i] < 0 && c[u][i] - flow[u][i] > 0)

				{

					pre[i] = u;

					min_flow[i] = min(min_flow[u], c[u][i] - flow[u][i]);

					Q.push(i);

				}

		}



		if (pre[m] == -1) break;



		for (int v = m; v != 1; v = pre[v])

		{

			u = pre[v];

			flow[u][v] += min_flow[m];

			flow[v][u] -= min_flow[m];

		}



		ans += min_flow[m];

	}



	return ans;

}



int main()

{

	int x, y, z;



	while (scanf("%d%d", &n, &m) != EOF)

	{

		memset(c, 0, sizeof(c));

		for (int i = 0; i < n; i++)

		{

			scanf("%d%d%d", &x, &y, &z);

			c[x][y] += z;

		}



		printf("%d\n", max_flow());

	}

	return 0;

}