[USACO4.2]Drainage Ditches

练习网络流的好题目，这里给出的是Dinic

#include<bits/stdc++.h>
using namespace std;
const int N=250;
int n,m;
struct Dinic{
    int G[N][N],dis[N];
    void build(){
        memset(G,0,sizeof G);
        for(int i=1;i<=m;++i){
            int u,v,f;
            scanf("%d%d%d",&u,&v,&f);
            G[u][v]+=f;
        }
    }
    bool BFS(){
        queue<int> Q;
        memset(dis,-1,sizeof dis);
        dis[1]=0;
        Q.push(1);
        while(!Q.empty()){
            int u=Q.front();Q.pop();
            if(u==n)return 1;
            for(int i=1;i<=n;++i){
                if(~dis[i]||G[u][i]<=0)continue;
                dis[i]=dis[u]+1;
                Q.push(i);
            }
        }
        return 0;
    }
    int dfs(int u,int low){
        int a=0;
        if(u==n)return low;
        for(int i=1;i<=n;++i)
            if(G[u][i]>0&&dis[i]==dis[u]+1){
                a=dfs(i,min(low,G[u][i]));
                G[u][i]-=a;
                G[i][u]+=a;
                if(a)return a;
            }
        return 0;
    }
    int dinic(){
        int ans=0,t;
        while(BFS())
            while(t=dfs(1,0x3f3f3f3f))ans+=t;
        return ans;
    }
}D;
int main(){
    while(~scanf("%d%d",&m,&n)){
        D.build();
        printf("%d\n",D.dinic());
    }
    return 0;
}