POJ 2375

BFS+强连通。输出max（缩点后出度为0的点数，缩点后入度为0的点数）。

 #include <cstdio>
 #include <iostream>
 #include <cstring>
 #include <cctype>
 #include <algorithm>
 #define LL unsigned __int64
 using namespace std; 

 const int N= ;

 struct Edge{
     int u,v;
     int next;
 }edge[N*];
 int tot,index;
 int head[N],dfn[N],low[N],stack[N],st;
 int outdegree[N],indegree[N];
 int belong[N],beg;
 bool instack[N];
 int map[][];

 int dir[][]={
     {,},
     {,-},
     {,},
     {-,}
 };

 void addedge(int u,int v){
     edge[tot].u=u;
     edge[tot].v=v;
     edge[tot].next=head[u];
     head[u]=tot++;
 }

 void Tarjan(int u) {
     dfn[u]=low[u]=++index;
     stack[st++]=u;
     instack[u]=true;
     for(int e=head[u];e!=-;e=edge[e].next){
         int v=edge[e].v;
         if (dfn[v]==-) {
             Tarjan(v) ;
             low[u] = min(low[u], low[v]) ;
         }
         else if (instack[v]) {
             low[u] = min(low[u], dfn[v]) ;
         }
     }
     int v;
     if (dfn[u] == low[u]) {
         beg++;
         outdegree[beg]=indegree[beg]=;
         do{
             v=stack[--st];
             belong[v]=beg;
             instack[v]=false;
         }while(u!= v);
     }
 } 

 bool ok(int i,int j,int x,int y){
     if(i>=&&i<x&&j>=&&j<y) return true;
     return false;
 }

 int main(){
     int x,y;
     while(scanf("%d%d",&y,&x)!=EOF){
         for(int i=;i<x;i++){
             for(int j=;j<y;j++)
             scanf("%d",&map[i][j]);
         }

         tot=index=st=beg=;
         int u,v,tx,ty;
         int spoint=x*y;
         for(int i=;i<=spoint;i++){
             head[i]=dfn[i]=low[i]=belong[i]=-;
             instack[i]=false;
         }

         for(int i=;i<x;i++){
             for(int j=;j<y;j++){
                 u=i*y+j;
                 for(int k=;k<;k++){
                     tx=i+dir[k][];
                     ty=j+dir[k][];
                     if(ok(tx,ty,x,y)){
                         if(map[tx][ty]<=map[i][j]){
                             v=tx*y+ty;
                             addedge(u,v);
                         }
                     }
                 }
             }
         }

         for(int i=;i<spoint;i++){
             if(dfn[i]==-){
                 Tarjan(i);
             }
         }

         for(int i=;i<spoint;i++){
             u=i;
             for(int e=head[u];e!=-;e=edge[e].next){
                 v=edge[e].v;
                 if(belong[u]!=belong[v]){
                     outdegree[belong[u]]++;
                     indegree[belong[v]]++;
                 }
             }
         }

         int ou=,in=;
         if(beg==){
             puts("");
             continue;
         }
         for(int i=;i<=beg;i++){
             if(outdegree[i]==){
                 ou++;
             }
             if(indegree[i]==)
             in++;
         }

         printf("%d\n",max(ou,in));
     }
     return ;
 }