POJ 1273 Drainage Ditches(最大流入门)

POJ 1273 Drainage Ditches(最大流入门)

http://poj.org/problem?id=1273

题意:

现在有m个池塘(从1到m开始编号,1为源点,m为汇点),及n条有向水渠,给出这n条水渠所连接的点和所能流过的最大流量，求从源点到汇点能流过的最大流量。

分析:

裸的求1点到M点最大流的问题,直接用Dinic算法即可.

AC代码:

#include<cstdio>
#include<cstring>
#include<queue>
#define INF 1e9
using namespace std;
const int maxn=200+5;

struct Edge
{
    int from,to,cap,flow;
    Edge(){}
    Edge(int f,int t,int c,int flow):from(f),to(t),cap(c),flow(flow){}
};

struct Dinic
{
    int n,m,s,t;
    vector<Edge> edges;
    vector<int> G[maxn];
    bool vis[maxn];
    int cur[maxn];
    int d[maxn];

    void init(int n,int s,int t)
    {
        this->n=n, this->s=s, this->t=t;
        edges.clear();
        for(int i=1;i<=n;i++) G[i].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;
        d[s]=0;
        Q.push(s);
        vis[s]=true;
        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]=true;
                    Q.push(e.to);
                    d[e.to]= 1+d[x];
                }
            }
        }
        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 flow=0;
        while(BFS())
        {
            memset(cur,0,sizeof(cur));
            flow += DFS(s,INF);
        }
        return flow;
    }
}DC;

int main()
{
    int n,m;
    while(scanf("%d%d",&m,&n)==2)
    {
        DC.init(n,1,n);
        while(m--)
        {
            int u,v,w;
            scanf("%d%d%d",&u,&v,&w);
            DC.AddEdge(u,v,w);
        }
        printf("%d\n",DC.Maxflow());
    }
    return 0;
}