SGU438 The Glorious Karlutka River =)（最大流）

  1 #include<cstdio>
  2 #include<cstring>
  3 #include<queue>
  4 #include<algorithm>
  5 using namespace std;
  6 #define INF (1<<30)
  7 #define MAXN 11111
  8 #define MAXM 666666
  9 
 10 struct Edge{
 11     int v,cap,flow,next;
 12 }edge[MAXM];
 13 int vs,vt,NE,NV;
 14 int head[MAXN];
 15 
 16 void addEdge(int u,int v,int cap){
 17     edge[NE].v=v; edge[NE].cap=cap; edge[NE].flow=0;
 18     edge[NE].next=head[u]; head[u]=NE++;
 19     edge[NE].v=u; edge[NE].cap=0; edge[NE].flow=0;
 20     edge[NE].next=head[v]; head[v]=NE++;
 21 }
 22 
 23 int level[MAXN];
 24 int gap[MAXN];
 25 void bfs(){
 26     memset(level,-1,sizeof(level));
 27     memset(gap,0,sizeof(gap));
 28     level[vt]=0;
 29     gap[level[vt]]++;
 30     queue<int> que;
 31     que.push(vt);
 32     while(!que.empty()){
 33         int u=que.front(); que.pop();
 34         for(int i=head[u]; i!=-1; i=edge[i].next){
 35             int v=edge[i].v;
 36             if(level[v]!=-1) continue;
 37             level[v]=level[u]+1;
 38             gap[level[v]]++;
 39             que.push(v);
 40         }
 41     }
 42 }
 43 
 44 int pre[MAXN];
 45 int cur[MAXN];
 46 int ISAP(){
 47     bfs();
 48     memset(pre,-1,sizeof(pre));
 49     memcpy(cur,head,sizeof(head));
 50     int u=pre[vs]=vs,flow=0,aug=INF;
 51     gap[0]=NV;
 52     while(level[vs]<NV){
 53         bool flag=false;
 54         for(int &i=cur[u]; i!=-1; i=edge[i].next){
 55             int v=edge[i].v;
 56             if(edge[i].cap!=edge[i].flow && level[u]==level[v]+1){
 57                 flag=true;
 58                 pre[v]=u;
 59                 u=v;
 60                 //aug=(aug==-1?edge[i].cap:min(aug,edge[i].cap));
 61                 aug=min(aug,edge[i].cap-edge[i].flow);
 62                 if(v==vt){
 63                     flow+=aug;
 64                     for(u=pre[v]; v!=vs; v=u,u=pre[u]){
 65                         edge[cur[u]].flow+=aug;
 66                         edge[cur[u]^1].flow-=aug;
 67                     }
 68                     //aug=-1;
 69                     aug=INF;
 70                 }
 71                 break;
 72             }
 73         }
 74         if(flag) continue;
 75         int minlevel=NV;
 76         for(int i=head[u]; i!=-1; i=edge[i].next){
 77             int v=edge[i].v;
 78             if(edge[i].cap!=edge[i].flow && level[v]<minlevel){
 79                 minlevel=level[v];
 80                 cur[u]=i;
 81             }
 82         }
 83         if(--gap[level[u]]==0) break;
 84         level[u]=minlevel+1;
 85         gap[level[u]]++;
 86         u=pre[u];
 87     }
 88     return flow;
 89 }
 90 int x[55],y[55],c[55];
 91 int n,m,d,w;
 92 bool isok(int time){
 93     vs=n*time*2+1; vt=vs+1; NV=vt+1; NE=0;
 94     memset(head,-1,sizeof(head));
 95     addEdge(vs,n*time*2,m);
 96     for(int i=0; i<n; ++i){
 97         if(y[i]<=d){
 98             for(int t=0; t<time; ++t) addEdge(n*time*2,i*time+t,m);
 99         }
100         if(w-y[i]<=d){
101             for(int t=0; t<time; ++t) addEdge(i*time+t+n*time,vt,m);
102         }
103     }
104     for(int i=0; i<n; ++i){
105         for(int t=0; t<time; ++t) addEdge(i*time+t,i*time+t+n*time,c[i]);
106         for(int t=0; t<time-1; ++t){
107             addEdge(i*time+t+n*time,i*time+t+1,m);
108             for(int j=0; j<n; ++j){
109                 if(i==j || y[i]>y[j]) continue;
110                 if((x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j])<=d*d) addEdge(i*time+t+n*time,j*time+t+1,m);
111             }
112         }
113     }
114     return ISAP()==m;
115 }
116 int main(){
117     scanf("%d%d%d%d",&n,&m,&d,&w);
118     for(int i=0; i<n; ++i){
119         scanf("%d%d%d",x+i,y+i,c+i);
120     }
121     if(d>=w){
122         putchar(‘1‘);
123         return 0;
124     }
125     int l=1,r=n+m+1;
126     while(l<r){
127         int mid=l+r>>1;
128         if(isok(mid)) r=mid;
129         else l=mid+1;
130     }
131     if(l==n+m+1) puts("IMPOSSIBLE");
132     else printf("%d",l+1);
133     return 0;
134 }