hdu

http://acm.hdu.edu.cn/showproblem.php?pid=1532

求最大的流量,用dinic算法就好。

// Rujia Liu
// 因为图较大，所以采用Dinic而不是EdmondsKarp
// 得益于接口一致性，读者无须理解Dinic就能使用它。
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>
using namespace std;

const int maxn = 50*50+10;

const int INF = 1000000000;

struct Edge
{
  int from, to, cap, flow;
};

bool operator < (const Edge& a, const Edge& b)
{
  return a.from < b.from || (a.from == b.from && a.to < b.to);
}

struct Dinic
{
    int n, m, s, t;
    vector<Edge> edges;    // 边数的两倍
    vector<int> G[maxn];   // 邻接表，G[i][j]表示结点i的第j条边在e数组中的序号
    bool vis[maxn];        // BFS使用
    int d[maxn];           // 从起点到i的距离
    int cur[maxn];         // 当前弧指针

    void init(int n)
    {
        for(int i = 0; i < n; i++) G[i].clear();
        edges.clear();
    }

    void AddEdge(int from, int to, int cap)
    {
        edges.push_back((Edge){from, to, cap, 0});
        edges.push_back((Edge){to, from, 0, 0});
        m = edges.size();
        G[from].push_back(m-2);
        G[to].push_back(m-1);
    }

    bool BFS()
    {
        memset(vis, 0, sizeof(vis));
        queue<int> Q;
        Q.push(s);
        vis[s] = 1;
        d[s] = 0;
        while(!Q.empty())
        {
            int x = Q.front(); Q.pop();
            for(int i = 0; i < G[x].size(); i++)
            {
                Edge& e = edges[G[x][i]];
                if(!vis[e.to] && e.cap > e.flow)
                {
                    vis[e.to] = 1;
                    d[e.to] = d[x] + 1;
                    Q.push(e.to);
                }
            }
        }
        return vis[t];
    }

    int DFS(int x, int a)
    {
        if(x == t || a == 0) return a;
        int flow = 0, f;
        for(int& i = cur[x]; i < G[x].size(); i++)
        {
            Edge& e = edges[G[x][i]];
            if(d[x] + 1 == d[e.to] && (f = DFS(e.to, min(a, e.cap-e.flow))) > 0)
            {
                e.flow += f;
                edges[G[x][i]^1].flow -= f;
                flow += f;
                a -= f;
                if(a == 0) break;
            }
        }
        return flow;
    }

    int Maxflow(int s, int t)
    {
        this->s = s; this->t = t;
        int flow = 0;
        while(BFS())
        {
            memset(cur, 0, sizeof(cur));
            flow += DFS(s, INF);
        }
        return flow;
    }
};

Dinic g;

int main()
{
  //  freopen("a.txt","r",stdin);
    int n,m,a,b,c;
    while(~scanf("%d%d",&m,&n))
    {
        g.init(n);
        for(int i=0;i<m;i++)
        {
            scanf("%d%d%d",&a,&b,&c);
            g.AddEdge(a,b,c);
        }
        printf("%d
",g.Maxflow(1, n));
    }
    return 0;
}