poj 1815 Friendship 字典序最小+最小割

  1 #include<iostream>
  2 #include<cstdio>
  3 #include<cstring>
  4 #include<cstdlib>
  5 #include<cmath>
  6 #include<algorithm>
  7 #include<vector>
  8 #include<queue>
  9 #define inf 0x7fffffff
 10 using namespace std;
 11 const int maxn=500,M=100000;
 12 
 13 int n,from,to;
 14 struct node
 15 {
 16     int v,flow;
 17     int next;
 18 } edge[M*4],save[M*4];
 19 int head[maxn],edgenum;
 20 
 21 void add(int u,int v,int flow)
 22 {
 23     edge[edgenum].v=v ;edge[edgenum].flow=flow ;
 24     edge[edgenum].next=head[u] ;head[u]=edgenum++;
 25 
 26     edge[edgenum].v=u ;edge[edgenum].flow=0;
 27     edge[edgenum].next=head[v] ;head[v]=edgenum++;
 28 }
 29 
 30 int d[maxn];
 31 int bfs()
 32 {
 33     memset(d,0,sizeof(d));
 34     d[from]=1;
 35     queue<int> Q;
 36     Q.push(from);
 37     while (!Q.empty())
 38     {
 39         int u=Q.front() ;Q.pop() ;
 40         for (int i=head[u] ;i!=-1 ;i=edge[i].next)
 41         {
 42             int v=edge[i].v;
 43             if (!d[v] && edge[i].flow)
 44             {
 45                 d[v]=d[u]+1;
 46                 Q.push(v);
 47                 if (v==to) return 1;
 48             }
 49         }
 50     }
 51     return 0;
 52 }
 53 
 54 int dfs(int u,int flow)
 55 {
 56     if (u==to || flow==0) return flow;
 57     int cap=flow;
 58     for (int i=head[u] ;i!=-1 ;i=edge[i].next)
 59     {
 60         int v=edge[i].v;
 61         if (d[v]==d[u]+1 && edge[i].flow)
 62         {
 63             int x=dfs(v,min(cap,edge[i].flow));
 64             edge[i].flow -= x;
 65             edge[i^1].flow += x;
 66             cap -= x;
 67             if (cap==0) return flow;
 68         }
 69     }
 70     return flow-cap;
 71 }
 72 
 73 int dinic()
 74 {
 75     int ans=0;
 76     while (bfs()) ans += dfs(from,inf);
 77     return ans;
 78 }
 79 
 80 int an[maxn][maxn];
 81 int main()
 82 {
 83     while(scanf("%d%d%d",&n,&from,&to)!=EOF)
 84     {
 85         bool flag=false;
 86         for(int i=1;i<=n;i++)
 87         {
 88             for(int j=1;j<=n;j++)
 89             {
 90                 scanf("%d",&an[i][j]);
 91             }
 92         }
 93         if(an[from][to])
 94         {
 95             printf("NO ANSWER!\n");
 96             continue;
 97         }
 98         memset(head,-1,sizeof(head));
 99         edgenum=0;
100         for (int i=1 ;i<=n ;i++)
101         {
102             if (i!=from && i!=to)
103                 add(i,i+n,1);
104             for (int j=1 ;j<=n ;j++)
105             {
106                 if (an[i][j]&&i!=j)
107                 {
108                     if (i==from)
109                         add(i,j,inf);
110                     else if (i!=to)
111                         add(i+n,j,inf);
112                 }
113             }
114         }
115         memcpy(save,edge,sizeof(node)*edgenum);
116         int ans=dinic();
117         printf("%d\n",ans);
118         for (int i=1 ;i<=n ;i++)
119         {
120             if (i!=from && i!=to)
121                 for (int j=head[i] ;j!=-1 ;j=edge[j].next)
122                 {
123                     if (edge[j].flow==0 && edge[j].v==i+n)
124                     {
125                         save[j].flow=save[j^1].flow=0;
126                         memcpy(edge,save,sizeof(node)*edgenum);
127                         if (dinic()!=ans-1)
128                         {
129                             save[j].flow=1;
130                             save[j^1].flow=0;
131                             continue;
132                         }
133                         if (flag)
134                             printf(" ");
135                         else
136                             flag=true;
137                         printf("%d",i);
138                         ans--;
139                         break;
140                     }
141                 }
142         }
143         printf("\n");
144     }
145     return 0;
146 }