UVa1658 Admiral（拆点法+最小费用流）

#include<cstdio>
#include<cstring>
#include<queue>
#include<vector>
#define FOR(a,b,c) for(int a=(b);a<(c);a++)
using namespace std;

const int maxn = 4000+10;
const int INF = 1e9;

struct Edge{ int u,v,cap,flow,cost;
};

struct MCMF {
int n,m,s,t;
int inq[maxn],a[maxn],d[maxn],p[maxn];
vector<int> G[maxn];
vector<Edge> es;

void init(int n) {
this->n=n;
es.clear();
for(int i=0;i<n;i++) G[i].clear();
}
void AddEdge(int u,int v,int cap,int cost) {
es.push_back((Edge){u,v,cap,0,cost});
es.push_back((Edge){v,u,0,0,-cost});
m=es.size();
G[u].push_back(m-2);
G[v].push_back(m-1);
}

bool SPFA(int s,int t,int flowlimit,int& flow,int& cost) {
for(int i=0;i<n;i++) d[i]=INF;
memset(inq,0,sizeof(inq));
d[s]=0; inq[s]=1; p[s]=0; a[s]=INF;
queue<int> q; q.push(s);
while(!q.empty()) {
int u=q.front(); q.pop(); inq[u]=0;
for(int i=0;i<G[u].size();i++) {
Edge& e=es[G[u][i]];
int v=e.v;
if(e.cap>e.flow && d[v]>d[u]+e.cost) {
d[v]=d[u]+e.cost;
p[v]=G[u][i];
a[v]=min(a[u],e.cap-e.flow);        //min(a[u],..)
if(!inq[v]) { inq[v]=1; q.push(v);
}
}
}
}
if(d[t]==INF) return false;
if(flow+a[t] > flowlimit) a[t] = flowlimit-flow;
flow+=a[t] , cost+=a[t]*d[t];
for(int x=t; x!=s; x=es[p[x]].u) {
es[p[x]].flow+=a[t]; es[p[x]^1].flow-=a[t];
}
return true;
}
int Mincost(int s,int t,int flowlimit,int& cost) {
int flow=0; cost=0;
while(flow<flowlimit && SPFA(s,t,flowlimit,flow,cost)) ;
return flow;
}
} mc;

int n,m;

int main() {
while(scanf("%d%d",&n,&m)==2) {
mc.init(n+n);
int u,v,w;
FOR(i,0,m) {
scanf("%d%d%d",&u,&v,&w);
u--,v--;
mc.AddEdge(n+u,v,1,w);
}
FOR(i,0,n) mc.AddEdge(i,n+i,1,0);
int cost,flow;
flow=mc.Mincost(n+0,n-1,2,cost);
printf("%d\n",cost);
}
return 0;
}