Drainage Ditches

Drainage Ditches

题目链接：http://poj.org/problem?id=1273

网络流

网络流模板题，学习网络流

SAP时间复杂度为O(V*E^2)，代码如下：

#include <cstdio>
#include <iostream>
#include <cstring>
#include <vector>
#include <queue>
#define N 205
using namespace std;
typedef long long LL;
LL const inf=1000000000000;
LL mp[N][N];
LL n,m;
bool vis[N];
LL pre[N];
void init(){
    memset(mp,0,sizeof(mp));
    for(LL i=0;i<n;++i){
        LL u,v,c;
        cin>>u>>v>>c;
        mp[u][v]+=c;
    }
}
LL bfs(LL s,LL t){
    memset(vis,0,sizeof(vis));
    memset(pre,-1,sizeof(pre));
    queue<LL>q;
    LL flow=inf;
    q.push(s);
    while(!q.empty()){
        LL p=q.front();q.pop();
        vis[p]=1;
        if(p==t)break;
        for(LL i=1;i<=m;++i){
            if(!vis[i]&&mp[p][i]!=0){
                flow=min(flow,mp[p][i]);
                pre[i]=p;
                q.push(i);
            }
        }
    }
    if(pre[t]==-1)return 0;
    return flow;
}
void updata(LL t,LL flow){
    for(LL a=t;pre[a]!=-1;a=pre[a]){
        LL p=pre[a];
        mp[p][a]-=flow;
        mp[a][p]+=flow;
    }
}
LL getflow(LL s,LL t){
    LL maxflow=0,flow=0;
    while(1){
        flow=bfs(s,t);
        updata(t,flow);
        maxflow+=flow;
        if(flow==0)break;
    }
    return maxflow;
}
int main(void){
    while(cin>>n>>m){
        init();
        cout<<getflow(1,m)<<endl;
    }
}

struct Edge{
    int from,to,cap,flow;
    Edge(int _from=0,int _to=0,int _cap=0,int _flow=0){
        from=_from;to=_to;cap=_cap;flow=_flow;
    }
};
vector<Edge>e;
vector<int>adj[207];
int a[207],p[207];//a[i]为可增广的权值
void addEdge(int from,int to,int cap){
    adj[from].push_back(e.size());
    adj[to].push_back(e.size()+1);
    e.push_back(Edge(from,to,cap,0));
    e.push_back(Edge(to,from,0,0));
}
int MaxFlow(int s,int t){
    int flow=0;
    while(1){
        queue<int>q;//findFlow
        q.push(s);
        memset(a,0,sizeof(a));
        a[1]=INF;
        while(!q.empty()){
            int u=q.front();q.pop();
            for(int i=0;i<adj[u].size();i++){
                Edge ee=e[adj[u][i]];
                int v=ee.to;
                if(!a[v]&&ee.cap>ee.flow){
                    a[v]=min(a[u],ee.cap-ee.flow); 
                    p[v]=adj[u][i];//记录边的序号
                    q.push(v);
                }
            }
            if(a[t])break;
        }
        if(!a[t])break;
        for(int i=t;i!=1;i=e[p[i]].from){//updata残余网络
            e[p[i]].flow+=a[t];
            e[p[i]^1].flow-=a[t];
        }
        flow+=a[t];
    }
    return flow;
}