[COGS746] [网络流24题] 骑士共存

 1 #include<cstdio>
 2 #include<cstring>
 3 const int maxn=800;
 4 const int maxs=maxn*maxn;
 5 inline int min_(int x,int y){return x<y?x:y;}
 6 int n,m,s,t,ans;
 7 int a,b,nh,nl;
 8 int map[maxn][maxn];
 9 int hb[]={-2,-1,1,2,2,1,-1,-2};
10 int lb[]={1,2,2,1,-1,-2,-2,-1};
11 int h[maxs],hs=1,ew[maxs],et[maxs],en[maxs];
12 void add(int u,int v){
13     ++hs,ew[hs]=1,et[hs]=v,en[hs]=h[u],h[u]=hs;
14     ++hs,ew[hs]=0,et[hs]=u,en[hs]=h[v],h[v]=hs;
15 }
16 int q[maxs],head,tail;
17 int d[maxs];
18 void bfs(){
19     memset(d,0,sizeof(d));
20     head=tail=0;
21     q[head++]=s,d[s]=1;
22     while(head>tail){
23         a=q[tail++];
24         for(int i=h[a];i;i=en[i])
25         if(!d[et[i]]&&ew[i]){
26             d[et[i]]=d[a]+1;
27             if(et[i]==t) return;
28             q[head++]=et[i];
29         }
30     }
31 }
32 int ap(int k,int w){
33     if(k==t) return w;
34     int dw=w;
35     for(int i=h[k];i&&dw;i=en[i])
36     if(ew[i]&&d[et[i]]==d[k]+1){
37         int wt=ap(et[i],min_(w,ew[i]));
38         if(wt) ew[i]-=wt,ew[i^1]+=wt,dw-=wt;
39         else d[et[i]]=0;
40     }
41     return w-dw;
42 }
43 void Dinic(){while(bfs(),d[t]) ans-=ap(s,ans);}
44 int main(){
45     freopen("knight.in","r",stdin);
46     freopen("knight.out","w",stdout);
47     scanf("%d%d",&n,&m);
48     s=0,t=1,ans=n*n-m;
49     for(int i=1;i<=n;i++)
50     for(int j=1;j<=n;j++)
51     map[i][j]=i&1^j&1;
52     for(int i=1;i<=m;i++){
53         scanf("%d%d",&a,&b);
54         map[a][b]=2;
55     }
56     for(int i=1;i<=n;i++)
57     for(int j=1;j<=n;j++)
58     if(!map[i][j]){
59         add(s,i*n+j);
60         for(int k=0;k<8;k++){
61             nh=i+hb[k],nl=j+lb[k];
62             if(nh>0&&nh<=n&&nl>0&&nl<=n&&map[nh][nl]==1)
63             add(i*n+j,nh*n+nl);
64         }
65     }
66     else if(map[i][j]==1) add(i*n+j,t);
67     Dinic();
68     printf("%d\n",ans);
69     return 0;
70 }