POJ2391 Ombrophobic Bovines 二分+最大流dinic

//floyd+二分答案+最大流判断 //但是一直wa，改天再详细查查原因。 //后来查出来是构图原因，修改的地方已经在注释中。 //现在AC了。 //我觉得代码写得还算容易看懂吧。 #include<iostream> using namespace std; #define MIN(a,b) (a<b?a:b) const int N=205; int n,m; int cow[N],shelt[N]; long long mat[N][N]; const int inf=1<<29; const long long INF=1ll<<59; struct Edge { int v,next,re; int w; }edge[N*N*2]; int edgehead[2*N]; int k=1; int final; int level[N*2]; bool visit[N*2]; int que[2*N]; void addedge(int u,int v,int w) { edge[k].v=v; edge[k].re=k+1; edge[k].w=w; edge[k].next=edgehead[u]; edgehead[u]=k++; edge[k].v=u; edge[k].w=0; edge[k].re=k-1; edge[k].next=edgehead[v]; edgehead[v]=k++; } long long floyd() { long long ret=0; for(int k=1;k<=n;k++) for(int i=1;i<=n;i++) for(int j=1;j<=n;j++) { if(mat[i][j]>mat[i][k]+mat[k][j]) { mat[i][j]=mat[i][k]+mat[k][j]; } if(mat[i][j]>ret&&mat[i][j]!=INF) ret=mat[i][j]; } return ret; } bool bfs() { memset(visit,0,sizeof(visit)); memset(level,0,sizeof(level)); visit[0]=true; level[0]=0; int head=1,tail=1; que[tail++]=0; while(head<tail) { int now=que[head++]; if(now==final) { return true; } for(int i=edgehead[now];i;i=edge[i].next) { int v=edge[i].v; if(!visit[v]&&edge[i].w>0) { level[v]=level[now]+1; visit[v]=true; que[tail++]=v; } } } return false; } int dinic(int now,int sum) { if(now==final) return sum; int os=sum; for(int i=edgehead[now];i&∑>0;i=edge[i].next) { int v=edge[i].v; if(level[v]==level[now]+1&&edge[i].w>0) { int tmp=dinic(v,MIN(edge[i].w,sum)); edge[i].w-=tmp; edge[edge[i].re].w+=tmp; sum-=tmp; } } return os-sum; } bool make(long long len) { k=1; memset(edge,0,sizeof(edge)); memset(edgehead,0,sizeof(edgehead)); for(int i=1;i<=n;i++) for(int j=1;j<=n;j++)//for(int j=i;j<=n;j++) { if(mat[i][j]<=len) { addedge(i,n+j,inf); } } for(int i=1;i<=n;i++) { addedge(0,i,cow[i]); addedge(i+n,final,shelt[i]); } int ans=0; while(bfs()) { ans+=dinic(0,inf); } if(ans<cow[0]) { return false; } else { return true; } } int main() { while(scanf("%d%d",&n,&m)!=EOF) { memset(cow,0,sizeof(cow)); memset(shelt,0,sizeof(cow)); for(int i=1;i<=n;i++) for(int j=i+1;j<=n;j++) mat[i][j]=mat[j][i]=INF; final=2*n+1; for(int i=1;i<=n;i++) { scanf("%d%d",&cow[i],&shelt[i]); cow[0]+=cow[i]; } int from,to; long long dis; for(int i=1;i<=m;i++) { scanf("%d%d",&from,&to); scanf("%lld",&dis); if(mat[from][to]>dis) { mat[to][from]=mat[from][to]=dis; } } long long s=0,l; l=floyd()+1; long long ans=-1; while(s<l) { long long mid=(s+l)/2; if(make(mid)) { ans=l=mid; } else { s=mid+1; } } printf("%lld\n",ans); } return 0; }