POJ3184 Ikki's Story I - Road Reconstruction（最大流）

求一次最大流后，分别对所有满流的边的容量+1，然后看是否存在增广路。

  1 #include<cstdio>
  2 #include<cstring>
  3 #include<queue>
  4 #include<algorithm>
  5 using namespace std;
  6 #define INF (1<<30)
  7 #define MAXN 555
  8 #define MAXM 11111
  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(bool statue){
 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                     if(statue&&aug) return 1;
 64                     flow+=aug;
 65                     for(u=pre[v]; v!=vs; v=u,u=pre[u]){
 66                         edge[cur[u]].flow+=aug;
 67                         edge[cur[u]^1].flow-=aug;
 68                     }
 69                     //aug=-1;
 70                     aug=INF;
 71                 }
 72                 break;
 73             }
 74         }
 75         if(flag) continue;
 76         int minlevel=NV;
 77         for(int i=head[u]; i!=-1; i=edge[i].next){
 78             int v=edge[i].v;
 79             if(edge[i].cap!=edge[i].flow && level[v]<minlevel){
 80                 minlevel=level[v];
 81                 cur[u]=i;
 82             }
 83         }
 84         if(--gap[level[u]]==0) break;
 85         level[u]=minlevel+1;
 86         gap[level[u]]++;
 87         u=pre[u];
 88     }
 89     return flow;
 90 }
 91 int main(){
 92     int n,m,a,b,c;
 93     scanf("%d%d",&n,&m);
 94     vs=0; vt=n-1; NV=n; NE=0;
 95     memset(head,-1,sizeof(head));
 96     while(m--){
 97         scanf("%d%d%d",&a,&b,&c);
 98         addEdge(a,b,c);
 99     }
100     ISAP(0);
101     int cnt=0;
102     for(int i=0; i<NE; i+=2){
103         if(edge[i].flow!=edge[i].cap) continue;
104         ++edge[i].cap;
105         if(ISAP(1)) ++cnt;
106         --edge[i].cap;
107     }
108     printf("%d",cnt);
109     return 0;
110 }

POJ3184 Ikki's Story I - Road Reconstruction（最大流）