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= 250050;

struct Edge{
    int u,v;
    int next;
}edge[N*4];
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[510][510];

int dir[4][2]={
    {0,1},
    {0,-1},
    {1,0},
    {-1,0}
};

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!=-1;e=edge[e].next){
        int v=edge[e].v;
        if (dfn[v]==-1) {
            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]=0;
        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>=0&&i<x&&j>=0&&j<y) return true;
    return false;
}

int main(){
    int x,y;
    while(scanf("%d%d",&y,&x)!=EOF){
        for(int i=0;i<x;i++){
            for(int j=0;j<y;j++)
            scanf("%d",&map[i][j]);
        }
        
        tot=index=st=beg=0;
        int u,v,tx,ty;
        int spoint=x*y;
        for(int i=0;i<=spoint;i++){
            head[i]=dfn[i]=low[i]=belong[i]=-1;
            instack[i]=false;
        }
        
        for(int i=0;i<x;i++){
            for(int j=0;j<y;j++){
                u=i*y+j;
                for(int k=0;k<4;k++){
                    tx=i+dir[k][0];
                    ty=j+dir[k][1];
                    if(ok(tx,ty,x,y)){
                        if(map[tx][ty]<=map[i][j]){
                            v=tx*y+ty;
                            addedge(u,v);
                        }
                    }
                }
            }
        }
        
        for(int i=0;i<spoint;i++){
            if(dfn[i]==-1){
                Tarjan(i);
            }
        }
        
        for(int i=0;i<spoint;i++){
            u=i;
            for(int e=head[u];e!=-1;e=edge[e].next){
                v=edge[e].v;
                if(belong[u]!=belong[v]){
                    outdegree[belong[u]]++;
                    indegree[belong[v]]++;
                }
            }
        }
        
        int ou=0,in=0;
        if(beg==1){
            puts("0");
            continue;
        }
        for(int i=1;i<=beg;i++){
            if(outdegree[i]==0){
                ou++;
            }
            if(indegree[i]==0)
            in++;
        }
        
        printf("%d
",max(ou,in));
    }
    return 0;
}