网络流--最大流--POJ 1273 Drainage Ditches

风骨散人Chiam

发布于 2020-10-28 11:32:48

2970

发布于 2020-10-28 11:32:48

文章被收录于专栏：CSDN旧文

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Description

Every time it rains on Farmer John's fields, a pond forms over Bessie's favorite clover patch. This means that the clover is covered by water for awhile and takes quite a long time to regrow. Thus, Farmer John has built a set of drainage ditches so that Bessie's clover patch is never covered in water. Instead, the water is drained to a nearby stream. Being an ace engineer, Farmer John has also installed regulators at the beginning of each ditch, so he can control at what rate water flows into that ditch. Farmer John knows not only how many gallons of water each ditch can transport per minute but also the exact layout of the ditches, which feed out of the pond and into each other and stream in a potentially complex network. Given all this information, determine the maximum rate at which water can be transported out of the pond and into the stream. For any given ditch, water flows in only one direction, but there might be a way that water can flow in a circle.

Input

The input includes several cases. For each case, the first line contains two space-separated integers, N (0 <= N <= 200) and M (2 <= M <= 200). N is the number of ditches that Farmer John has dug. M is the number of intersections points for those ditches. Intersection 1 is the pond. Intersection point M is the stream. Each of the following N lines contains three integers, Si, Ei, and Ci. Si and Ei (1 <= Si, Ei <= M) designate the intersections between which this ditch flows. Water will flow through this ditch from Si to Ei. Ci (0 <= Ci <= 10,000,000) is the maximum rate at which water will flow through the ditch.

Output

For each case, output a single integer, the maximum rate at which water may emptied from the pond.

Sample Input

Sample Output

模板题，题意很明了，直接测板子。

#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,t;
    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;
}

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原始发表：2019/11/02 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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